There are three different issues here, and I think it's a good idea to make sure we all know we're talking about three different things.
1) Sexual orientation isn't something you can generally just "decide to do" because it's fashionable or trendy. Sexual BEHAVIOR is something you can decide to do for a number of reasons. BEHAVING in a particular way, sexually, does not necessarily have anything to do with your ORIENTATION.
For instance, you can be heterosexual and choose to be celibate (not have sex with any partner in any way). Does that change the fact that you are heterosexual, simply because you are not having sex with a partner? No way. Or you could be a lesbian and choose to be celibate. Does that change the fact that you are a lesbian? Similarly, no.
Or you can identify as heterosexual -- your primary sexual thoughts and impulses could be about people of the opposite sex -- and yet engage in same-sex sexual activity. This often happens, for instance, in prisons, where there are only people of one sex. Do those people think of themselves as bisexual or gay? No. They still think of themselves as heterosexual, even if for some period of time their sexual BEHAVIOR is same-sex behavior. Does it change their fantasies and their thoughts about sex? Generally NOT, they still think about opposite-sex partners, often even WHILE they are engaging in same-sex sexual activity.
Human beings are very versatile animals. Like many other mammals, they are capable of engaging in all sorts and kinds of sexual activities. The gender of the sexual partner is more or less irrelevant -- biologically speaking, our sexual anatomies function just fine in sexual acts with people of any gender.
BEHAVIOR does not necessarily have anything to do with IDENTITY.
2) A threesome does not equal bisexuality, gayness, or lesbianism. It does not even equal same-sex sexual activity.
A threesome is merely sexual activity with three people present. A threesome does not imply same-sex contact at all. Assuming that there are two people of one sex and one person of another in a given threesome, that STILL does not mean that the two people of one sex have any sexual contact with ONE ANOTHER. There are no laws that say that any two people in ANY group of three MUST have interactions with one another. There are several available permutations. Think about it. In short, it is very possible that a sexual threesome might involve NO sam-sex sexual activity at all.
Also, bear in mind that mixed-gender threesomes are not the only kind of threesomes that are possible. Three people of the same gender can ALSO have a threesome... nothin' bisexual, even behaviorally, about that one at all either.
3) People experiment sexually for a lot of reasons. Sometimes it's to get attention, sometimes it's out of genuine curiosity, sometimes (like in the prison situation I mentioned above) people might do something they wouldn't normally do because it's the only way they can have sexual intimacy and closeness in a given environment.
Assuming that people are experimenting with sexual alternatives because it's "fashionable" or because they're just looking for attention is not necessarily WRONG, but it isn't necessarily RIGHT, either.
And even if they are experimenting, so what? It doesn't tell you anything substantial about who a person is, or even necessarily what they've done sexually.
And even if it did, it'd be NONE of your business. Butting your nose into other people's sex lives is rude. It's eminently possible to be a good, honest, upstanding, worthwhile, intelligent human being and have an unconventional sex life... so if you operate on the assumption that people are *people* first and deserve to have their private lives be just that, private, you don't go rooting around unasked in other people's business.
Start a Revolution -- Stop Hating Your Body!