I believe this was from the 90’s. Could have been really late 80’s maybe really early 2000’s.  I didnt see entire film.  Went looking for it later and could never find it. Remember young man and woman were at older mans house.   I believe it was young mans estranged father but could have just been older man. Young woman is with the older man.  They are at table and young man is trying to touch the young woman and she is trying to stop him with out older man finding out.  Later everyone is asleep except young man.  Young woman comes out of bedroom kind of frustrated and goes to young man straddles him in chair and they are having sex.  Older man hears and catches them, chases them out of the house.  They run away taking car and go on road trip.  Later in movie they end up in a cavern/cave with a pool were they are playing around.  Any suggestions would be appreciated.  Thanks