An important way that informed consent is relevant to sexual pursuits is in the question of relationship status, availability, and ethical entanglements.
Let me put it this way: say that you’re single and on a date with someone. It’s going well. What if, after the date (and whatever ensuing erotic activities you might’ve pursued), this person reveals that they’re actually in a long-term committed relationship, or married? How would you feel about that?
Now, if you’re already familiar with the concept of polyamory or open relationships (more info on those here and here), you might tilt your head, say, “Huh, okay,” and then want to talk more about what just happened.
But if you’re not, you might feel confused or hurt. Maybe you’d feel like that person led you on. A lot of folks go on dates in order to see if they’re compatible with the person in order to evaluate whether they’d like to be in a long-term relationship. And if the person in question is already “taken” – even if they’re available for sex or flings or some kind of relationship – then that could be a problem.
Maybe people shouldn’t assume upon going into a date or sexual encounter with a new person that they are, in fact, available, but American culture tends to accustom us to thinking in this manner. As in: you’re on a date with someone, thus you must be available. Some people, of course, pretend to be available but are actually practicing non-consensual non-monogamy (that is to say, cheating). Finding out that someone used you to cheat can feel like a nasty betrayal, and it can be. Then again, other people don’t mind being in situations related to infidelity.
People entering into relationships – whether sexual, romantic, some of the above, or none of the above – have a right to know what they’re getting into. Not everybody is going to be cool with hooking up with someone who already has a primary partner (regardless of whether it’s happening consensually on all fonts).
For people who want to date around or try open relationships but are afraid of the stigma that comes with making an alternative lifestyle choice, it’s better to be up front about your status at first and risk rejection than to draw someone in without their knowledge and in a way that puts them in an ethical dilemma. If nothing else, sleeping with someone who’s sexually active with other partners raises safer sex issues, not to forget about all the ways in which emotions and sex can become intertwined for some.