Relationships in this game are ALL optional. Go with whom you want, the ones you refuse, well, I don't know what happens, never refused anyone, but I imagine they stay in the game as friends or something like that.
Lisa jumped topless, because the top wasn't hers, she borrowed it from Emma, so her intention was clearly to not potentially ruin a top that wasn't hers. What makes a bit less sense is something you didn't pick up on, she took the time to take off the top, but, NOT her cell phone. So, she's a considerate airhead... well, we already knew that, didn't we?
Peace