When two straight guys are dared to kiss in front of other [most likely drunken] friends, one out of the two guys dared are usually always up to kiss the other guy... on the lips. But they fear rejection for wanting to experiment with guys and fear being labeled "gay" by their judgemental friends.

So when the one guy who wants to experiment is the first to initiate the male on male kiss with his buddy, he uses his hand to cover his friend's mouth so that he will kiss the back of his own hand thus pleasing the drunken friends watching and pleasing the other guy who may have not wanted the kiss at all. The other guy is safe from "homosexual nightmares" and the guy who really wanted to kiss will end up jerking off to gay porn later that night.

This is the Straight Male Behaviour that you should look out for that is known as the Hand Over Mouth Kiss.