Ummm... I think I have to disagree with other posters, who don't seem to think this is a big deal. It is. My husband came home and told me a story that his co-worker had told him. His co-worker's 12 year old son was participating in "smile parties." This is where several boys sit at a table and some girls are underneath giving oral sex. The first boy to smile loses the game. Cute, huh?
This is a serious thing. I really believe that boyfriends should not be acceptable at all--even the "I like you" kind of things until at least high school. Kids are becoming sexually active earlier and earlier, and I think we do need to talk to our kids, and discuss family values and expectations. Part of this is enforcing standards.
Honestly, if that was my kiddo, I would be terrified and probably yank her out of school to homeschool her. I would limit or eliminate a great deal of media--TV, movies, CD's, internet, books, etc... that encouraged that type of behavior as well. That may be a bit extreme, but that is my thought on the matter.
I really did like the book a previous poster mentioned, called Hold on to Your Kids by Neufeld. It was a great book and I think could be really helpful. I also think the suggestion of making sure you are giving your DD enough attention now that you have tiny twins is really important. Could you schedule Mom-daughter dates every so often for just the two of you to go out and talk about what is going on in her life?
It is a tough and really serious situation. I wish you the best as you seek guidance on dealing with it.