Coming from someone who's polyamorous, it's not that odd, actually.

Still a deceiving asshole even if he is in an open relationship. He should've told OP about it.

I'm polyamorous and I tell people pretty much upfront that I am. I have 2 girlfriends currently and still talk to others. While poly isn't a bad thing. It is not acceptable to hide it or keep it from someone you are talking to. ever.

I find it extremely sad that you even have to have this type of a conversation. It's kind of expected that if you're seeing someone then it's exclusive, unless it's been otherwise stated. I would expect anyone that is interested in dating more than one person, or have an open relation, to be open about that and not hide it. Seems extremely deceitful to me to not say anything.

I find it ridiculous to expect someone you just met to conform to your social mores. I don't expect someone to know what my social values are when I meet them. Monogamy may be an automatic for you, but if you assume it is for everyone else you are going to get burned and it is no ones fault but your own.

But how many dates is considered seeing someone.

I have no choice BUT to respect you for this honesty. I tell men FROM THE GATE that if either of us gets bored or wants out then we MUST GET OUT! I honestly have left because I'd rather break their heart ONCE then let them love me while I love someone else :) as a result I've loved TWO men in my life and I've NEVER cheated. So being up front is probably best that way the ladies you get involved with get it from go. No one can make you out to be a bad guy when they knew what you were about BEFORE they fell for you.....GOOD JOB ON THE HONESTY

Personally, I could understand if he'd withheld the info for a few dates, but I think if they'd gotten to the point where they were sleeping together, he should have mentioned it. Increased partners equals increased chances of catching something...even if they're all tested now and then, one might always skip/lie, they could catch something in between tests, etc...Of course most poly people are pretty cautious so it ends up being fine, but imo he should still have let her know so she could make the decision herself , even if there was no emotional attachment and it was just boning. Being poly is fine, but the vast majority are not and some people haven't even ever heard of them. It makes more sense for them to inform others at some point (maybe after a few dates, but before it gets emotionally or physically serious) rather than for everyone else to inform others of their monogamy. And honestly, if the only reason he didn't tell her was because he was knew she would disagree to sleep with him if she was aware, then I think it's pretty shitty of him to then withhold info he KNOWS would change her mind and then sleep with her anyways. If he just didn't think to mention it though, then that's another story.

I think if you're poly or into open relationships or dating someone else or whatever, then that's something that should be said on the first date. The fact that you just said, she probably wouldn't have gone out with him if she knew, is the exact reason why he should have told her the first time they met. It's not fair to try and trap someone into a relationship, and wait till they have an emotional connection before you bring something like that up. I think it should be brought up on the first date so the other person knows what they would be getting into and if it's worth their time and commitment. It's not fair to hold off on telling them. For me personally, I only want exclusivity and a monogamous relationship, so it's a waste of my time (& his) for me to bother going out with someone a few times if we don't have a future and don't have the same lifestyle.

No. Omitting the truth is its own form of deception. If this was a one-night-stand with no expectations of commitment, sure, but if you're seeing someone, you need to be upfront from the start about something like polyamory, and if you're not, you need to have the "should we be exclusive?" conversation before sex or by the third date, whichever happens first. The vast majority of people are monogamous, even though there's nothing wrong with being poly, and letting someone assume you are too when you're not can (exhibit A: this FML) rope them into something they aren't comfortable with.

And yeah. If you deliberately lie and misrepresent your situation to get consent that wouldn't be given otherwise, that's really shady and borderline nonconsensual. Don't trick people into sex they don't want to have.