Heather Corinna replies:
In fact, because there are extra fluids present, plenty of women (as well as some men) find some kinds of sex -- like vaginal intercourse -- more pleasant during menses. Women often will experience high levels of arousal during menstruation because of decreased estrogens in their bodies at that time, and for women who chart their fertility and know that during menses, they as individuals (remember, all our cycles vary somewhat) are far enough away from ovulation for that to be a low-risk time per pregnancy, their enjoyment if they have opposite-sex partners may be increased because of fewer concerns about pregnancy. In addition, via masturbation or partnered sex, sexual activity and orgasm can help reduce cramps and other menstrual symptoms during menstruation. At the very least, it's certainly a great distraction.
Can it be messy? Of course it can. But you know, sex is usually messy in some way. If not literally, certainly emotionally and intellectually!
With most kinds of sex, we're already dealing with fluids, and semen (and some lubricants) will often stain sheets or clothing, as will menstrual flow. But for a woman who wants to have genital sex during her period and make things less messy, she can simply put a towel beneath her that's easily washed or not as likely to show bloodstains. As well, women who want to have genital sex during menses but avoid the mess entirely can use a diaphragm (but not a cervical cap) or Instead menstrual cup to contain flow during sex without obstructing the vaginal opening or canal. Or, for a woman who isn't interested in vaginal sex, but in something like clitoral stimulation only, it's fine for her to have a tampon in during that sex (though since tampons absorb vaginal fluids, she'll likely need to use more extra lubricant from a bottle than usual). Just be sure not to try and have vaginal sex with a tampon in: that's neither wise nor comfortable.
Just so you don't get freaked out if this happens, because orgasm causes extra uterine contractions, sometimes right after sex during menses, it may appear that your menstrual flow has abruptly stopped, something that sometimes causes women to worry they because pregnant right then and there. But that's not what's going on: rather, those contractions can just push out extra flow more, which means you also may end sex seeing more flow on your genitals or your partners than you did when you started. You may even have pushed out the very last of your flow, so it's not abnormal in the last few days of a period to have sex, then find flow stops. Remember that pregnancy isn't instant: it's a process that takes around a week to complete: we can't instantly become pregnant and have it change our fertility processes in an hour.
Some women won't want to have sex during menses, and some partners may be squeamish. Sometimes, that's just because people can get freaked out by blood, which is pretty darn normal: after all, usually when we see blood it's because someone is hurt or injured, so it can tend to invoke a natural reaction of fear in us, especially if we weren't expecting to see it. Other times, that's because a person feels like there is something shameful or gross about menstruation in particular, which while common enough to call normal, tends to be based in a lack of acceptance of women's real bodies, as well as very archaic and uninformed ideas about women and menstruation. Those attitudes aren't just limited to women's sexual partners, either: some women may feel embarrassed about or ashamed of their periods, or just feel that menstruation is a time they'd rather spend alone for any number of reasons, from religious or spiritual beliefs to a preference to lay on the couch and watch movies rather than having sex.
Do be aware that because there is blood in menstrual flow, some sexually transmitted infection risks can be higher for partners during menstruation, particularly for bloodborne viruses like HIV or Hepatitis. So, while safer sex is always important, it can be especially important -- particularly with more casual partners or with new sexual partners -- for sex during menstruation.
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