Heather Corinna replies:
If that's truly the case -- if any two people have had NO sexual contact of any kind with other partners -- then, for the most part, either of them having a sexually transmitted infection is highly unlikely. It's never sage to just presume someone has no STIs without having had any testing, but in this situation, you can know that it's not a likely scenario with most STIs.
But there are some important caveats.
For starters, when we call an infection "sexually transmitted," we do so because that is how it is most often transmitted or contracted: not because that is only how it is transmitted or contracted. While many STIs are highly unlikely to be transmitted any other way than with oral, vaginal or anal sex -- like Chlamydia, Gonorrhea, Syphilis, genital Herpes or HPV -- some STIs can be and are transmitted or contracted nonsexually, such as by sharing sheets or towels, intravenous drug use, at birth or through breastfeeding.
HSV-I -- oral herpes -- is most commonly transmitted non-sexually, and most people who have it contract it in childhood. Oral herpes can be transmitted between those two people, orally or genitally. Given how many people have oral herpes, that is a considerable risk. But most people with oral herpes have had cold sores (the most common symptom of HSV-I) at some point, and if they don't know if they have, could certainly ask a parent if they remember them ever having one. As well, even if Herpes is transmitted, while it certainly is an inconvenience and no one wants it, in most people, it also is not likely to cause any serious health problems.
Some other sexually transmitted infections can be transmitted nonsexually. For example, while in many areas, most women are now being screened for HIV with their pregnancies, and many children getting regular health care will be tested for infections they could have acquired through their mothers well before their teen years, HIV can be transmitted through breast milk, so some people get HIV that way. HIV can also be transmitted through intravenous drug use or the use of a dirty needle (like with tattooing or piercing), as is also the case with Hepatitis. Pubic lice -- often colloquially called "crabs" -- and trichomoniasis are STIs that are often also transmitted through casual contact, like by sharing damp towels with someone who has one of those, rather than just through sexual contact.
The other big caveat is whether or not any two people really have or have not had any sexual partnership before. Two big issues with what we can know that are that a) many people are not truthful about their sexual history, even to people close to them and b) people often tend to define what is and isn't sex differently.
Let's face it: plenty of us know that sexual history can be hella loaded. Often when we're talking to someone about it, it feels like, or is clear that, there is a kind of answer they want and a kind they don't. In some situations, it feels precarious to say we've had any previous sexual experience, while in others, saying you've none is loaded: it depends on who's asking and what they think is ideal. People tend to make judgments about us based on sexual history -- whatever it is. And some partners clearly would like their partners to never have had others, particularly if they have not themselves. Plus, when we're talking about sex, it's usually because we want sex, and when any of us has an agenda, it can tend to be more likely to be dishonest if we think doing so might get us what we want.
For those reasons and more, many people find it very hard to be honest about sex and their sexual history, and many just plain won't be. Young people, in particular, also tend to have a pattern of being more likely to be dishonest about sexual history than older people do, which isn't all that surprising when you consider how much more loaded an issue sex often is for teens and young adults.
I also know that when we love and trust someone -- especially if we've known each other for some time or been dating for some time -- or even when we just really like someone, it's no fun to consider that that person may not be truthful about sexual history, but it is common. So, to be realistic, we have to acknowledge that's entirely possible. As people who work in sexuality research know, we have to be very careful about how much stock we put in sexual self-reporting, because it has a long history of being notoriously unreliable.
Lots of people define what sex is and isn't differently, too. We can ask twenty different people who have all had oral or anal sex, for instance, if they have had sex, and some will say they have while others will say they have not. Even for those who have had intercourse, some people won't "count" that as sex sometimes if, for instance, a penis was only in a vagina for a minute or two (despite that being typical for many first-times), not in a certain depth, if vaginal bleeding didn't happen, or even just if that person would have preferred that experience was not one they had.
Personally, I'm of the mind that the best way to deal with all of this when you are starting to become sexually active is just to start with safer sex right at the gate, just like you'd wear a seatbelt the first time you were driving in a car. Since most people will not have only one sexual partner for the whole of their lives, you and/or your partner are likely to have to learn to use safer sex in time anyway, so there's little sense not learning right from the start.
I also think that when you come to sex together as safely and responsibly as you can, that it not only protects your health as best as possible, but it strengthens your relationship and is one thing that can tend to enable the kind of environment where partners actually feel more able to be honest about their sexual history. If your partner happens to feel like asking for safer sex means you don't trust them, you can inform them of some of the issues I've filled you in on, and also let them know that people develop trust over time, not instantly. No one should be expected to trust blindly or right off the bat. We earn trust from people, gradually, based on behaviors like, for instance, two people showing each other that they care enough about both of their health to play things as safe as they can if they're going to be sexually active and to be sexually responsible. Trust, all by itself, just doesn't keep us physically safe or healthy.
Negotiating safer sex also makes negotiating birth control, if and when that is a factor easier, as well as negotiating sexual limits and boundaries or sexual desires much easier. In the case that talking about any of this or insisting on safer sex seems too daunting right now, it's probably a good idea to have another think on if you're really yet at a place -- in your life or your relationship -- where it's the right time for sex.
What I suggest anyone does is what is suggested by sound health organizations like the CDC and WHO when it comes to protecting yourselves as best you can if you're going to be sexually active. You should use latex barriers (like condoms) for all oral, vaginal and/or anal sex for the first six months of any relationship. You and your partner should each get tested during that time: if neither of you have had any sexual partnership at all before now (and none besides each other during that time), one full screening for both of you at the end of that six months will do just fine. You both also will need to be sexually exclusive during that period of time to assure no new risks are being introduced to the equation. In the meantime, you can know that using safer sex practices and it being possible neither of you likely has any STIs means you have very little risk, which lets the two of you actually enjoy the sex you're having more, physically and emotionally. Less stress = better sex.
If those tests come out negative, and if you both have stayed sexually exclusive and intend to remain so, then at that point, if you like, you can go without latex barriers as your STI risks will be very low -- and known to be, not presumed or guessed -- at that point. And then, too, you two will have had a substantial period of time together to build trust, and to be able to extend it when it comes to something like trusting that both of you are staying monogamous if you say that you are.
Okay? I know that sometimes safer sex and taking precautions can seem less ideal (even though the idea of sex with no precautions being more romantic is a bit of a strange ideal, since it often puts people at risks of some very unromantic things) or like a drag, but it really, truly only is as much of a drag as anyone makes it. When it is a regular habit for everyone involved, it's just no big deal -- no more so than other common aspects of sex like having to shift positions now and then because our legs hurt, taking a second to go pee, or asking a partner to change up what they're doing because something else feels better -- nor is it something that gets in the way of trust, intimacy or enjoyable sex. Rather, it tends to nurture those things a lot better than gambling with presumptions does.
Here are some more links for you to inform yourself with: