Do the girls actually start sexual stuff on their own without you engaging first? I had Jubilee start a blowjob scene on her own but we already had sex at that point.
Depends on how you choose to view it really.
One thing I've noticed is if you enter a girl's room after she's given you her key, if she's naked and randy enough there's a chance that you'll get a "Oh, hello there," before jumping straight into the naughty time menu (Keep in mind that a girl gives her key after a number of sleepovers, regardless of whether hanky and/or panky preceded sleepy time.) I also seem to recall dialogs involving invitations to the bedroom after class for some "biology lessons" or something like that.
So yeah, you can interpret that as girls taking the lead, or not. Your choice really.
Other that that I'm fairly certain that when Jean unilaterally declares you to be her sex friend, sex is non-optional afterwards. I mean, I do remember the unsanctioned morning blowies started after that event... but it has been some time since I last saw that one, so it may have been tweaked a bit since.