Shane is the most corruptible person in the group. If he was in charge of things, the others would be in as much danger from him as they would from zombies.
Serious question: Has Shane done anything bad (that we know of) that can't be explained? I mean sure he was nailing Bad Hair Sheriff Wife but he thought her husband was dead. And the thing with Otis, as terrible as we might see it, some might consider justified.
The sex thing was no big deal unless, of course, his heightened sense of survival leads him to future ill deeds because of his base animal desires. And that's the thing about Shane -- he's already decided that all of the old rules and all of the old moralities no longer apply. He'll take what he wants and worry about rationalizing it less and less as time goes on.If he ever achieves the leadership, he will seek no counsel from others, brook no dissent and tolerate no separation attempts. Not only will he lead them to disaster, he'll also be increasingly despised for his tyranny along the way. And he won't care as long as he comes out of it with his own hide and a woman or three.