Why? Why did they agree to do this?

Well, it was just a party like any other, a party at Clyde's house, but everyone drank too much, except Kyle, since he didn't like the taste of alcohol

And then some idiot came up with the idea of playing truth or dare, but without the truth part, so it's dare or dare, in short, you do whatever they ask without complaining

Which led to the current situation.

It was Cartman's turn to choose who he would give the challenge to, and of course... he chose Kyle

"I dare you to kiss someone in the circle!" The fat guy said, there was only one girl among them, Wendy, so everyone imagined that Kyle would kiss her.

"I'm going to kiss Stan." Kyle said clearly irritated by the challenge but willing to do it, everyone was shocked and Cartman had a smile on his face, clearly enjoying the situation.