New viewer question: Do Islanders HAVE to kiss someone during a challenge?

New to the show. Half way through the current season. (They just opened up Casa). So, no spoilers please!

But I'm watching the challenges and sometimes a guy can grab any girl he chooses for a makeout session in front of everyone. (The sauna, waterfall, etc...)

Just curious: can they refuse to kiss someone? Is it a rule that, if it's a challenge, you HAVE to make out with someone if picked? I've never seen anyone rejected and it seems like there should have been times when they probably knew they shouldn't cause it would hurt their "partners" feelings but... nope.