People who have no regard for boundaries. I mean, for real. I'd get pissed enough myself if a guy put his hands on me inappropriately without my permission. But women are terrorized every moment because some guy gets it in his head that he can take advantage of any lady who looks weak enough or is separated from the herd. Who in their right freaking mind ever thinks it's okay to block someone's path, touch, or even grope another person? Especially if it is a stranger or someone they barely know. I get that there's this "people are mingling" mentality at bars and social scenes, but damn people there are still obvious boundaries in place.
Decent people start with talking to the person. Try to engage a little bit, get to know them a little bit. Read their body language.
If they don't seem interested then don't bother them anymore let alone touch them. And if they seem more welcoming, there's always a hand shake, one armed hug, shoulder squeeze, whatever to have appropriate, personable touch. And guess what? If someone's drunk they're not your good time.
Here's something to relate: My mom got her butt groped at a bar once. She punched the guy on the top of his shoulder hard enough that even though he was still walking away he dropped to one knee. He didn't turn around and when he got back up he ran.
I'm not saying people should do that. Dudes are retarded and might confront or escalate. But sometimes its nice to have a creep get put in his place like that. And ladies shouldn't be afraid to be drinking and enjoying themselves in public ya know?