I know I wouldn't prefer my daughter playing that as a teen and would talk to her about it, but I don't think you can stop it because they did say they play it a lot. It can breed other worse behaviors, but sex and all that can be caused without the game. My main thing would be talking to her about fidelity in a relationship. I'm not sure how old she is or when she can date, but I would talk to her anyways to make sure she knows that in a relationship the boy and girl should be monogamous unless otherwise agreed and that this game is not an excuse for boyfriend kissing someone else. Also talking about the game in general, how she feels about playing it and what it can lead to (sex, wrong signals to someone, kissing someone who could already be sexual active and have syphallis or something). and say your opinions in a non-confrontational and non-judging way.
Maybe you could've started a water balloon fight or played kinect or rockband, teens seem to really like those games lol. I don't think that parents should just leave teens alone for a long time without checking on them, just because these situations can happen. While I know you can't stop it all the time you don't give them a hideout to do it or condone it. I'd say good job on your persistence. I'll have to research ways so my daughter doesn't get into that stuff when she's older lol. Good luck :)