Originally Posted by Olivier
Depending where you live, it's pretty common not to mention any partners you already have when you are dating someone for the first time. Only once the relationship becomes established you start setting the terms. In regards to sex: Practice safe sex.
I was assuming condoms already, but I would want to know first what the risks are. I would be upset if someone didn't disclose their other parters (or their STD/STI status if they are aware of a positive one) before we have sex, even though we'd use condoms either way, because if they have other partners I would probably want to make sure it's serious before having sex. I would want to meet their other partners and make sure they're all aware of me and fine with it. It's not just about the health status, it's also about being sure there is no cheating.
Yes, it might be possible to date around in some places, but the existing long-term partners are another story, and the new partners have the right to make sure they're not going to be involved in some drama. Not to mention they might not be willing to date someone if there is no monogamous future with them in the future, as distant as it might be.
In this situation the GF is just getting comfortable with the idea of being open, I think it would also be helpful to her to see that her boyfriend and his other girlfriends are respectful of that. I mean, she's just getting used to opening the relationship, the last thing she needs is knowing that her boyfriend doesn't even mention her until after sex has occurred a few times.
I think it's a matter of respect towards all your partners not to pretend they don't exist, because they deserve to know about one another. Even when dating around, you'd tell the person that you're dating around. I don't know, maybe it's a cultural thing, but the assumption for me has always been that you are single and only dating that one person, unless you tell them "I'm dating around".
In which case they might stop saying no to people, for instance. When I had several partners (although I didn't date any of them, it wasn't that serious) I always told them I had sex with other people. Then I waited to see what they thought. In a lot of cases, they switched from acting like a boyfriend to acting like a friend with benefits. In some, they would tell me they were not interested anymore.
I'm sure they would have felt misled if I had said nothing. I don't think you can assume that anyone you date is a virgin with no other experience, but I've never been in a context when you can expect them to be dating more than one person at a time.
Even if you don't say it on the first date, I would certainly make sure to mention it before any sex is involved. To a lot of people, sex and exclusivity go hand in hand.