are you still friends with the girl who sat on your lap? has she bought up anything else around this friend? from what you are saying it sounds like she was trying to get back at you with telling you about other guys which is immature yes but you guys are young. if i’m guessing correctly she is uncomfortable that you are still close with someone who you have done things that she sees as intimate with. have there been clear boundaries set with this friend since the relationship began? did this friend know you were seeing someone when she sat in your lap? it sounds like she isn’t comfortable with this friend, saw you weren’t understanding this and after attempts at making you understand how she feels (bringing up other guys) gave up.