You should definitely disclose your poly lifestyle to someone *before* you have sex with them. Usually I recommend you tell them on your first, second, or third date with them. Although as I said, tell them sooner if you are going to have sex with them sooner.
It completely depends on the situation. If you're out and about to hook up with someone almost like a one night stand, do you need to tell them beforehand? I don't think so. If it comes up, great. If it is really important to someone that they don't have sex with someone who is partnered, then it is on them to ask before such encounters.
I tend to make sure people know before we go on a date but not necessarily before sex.
Interesting responses so far. I’m curious about how everyone on the board approaches this so please chime in.
I think poly-orientation has more to do with relationships than sex. So it seems appropriate to begin talking about poly when you begin talking to someone about having a relationship together. Which for me is usually after sexual activities.
For me I'm not sure I *could* get to the point of sleeping with someone before disclosing poly etc - I have only ever been intimate with someone on the first date once, and even then it was someone I met off a dating app where I was *very* clear about my partner (at the time I only had one committed partner). Otherwise, it's historically been 2-4 dates before I slept with someone, and quite frankly I don't think it'd be possible to talk to someone for that long without mentioning my partners, they're too big a part of my life.