OK. I can understand where the OP is coming from in some ways.
1, not wanting to be sandwiched between two obese people....Not the kindest choice of words, but...yeah, being between two big people, whether it's big built like linebackers, or simply overweight, can be kind of uncomfortable.
2, I can even understand the having his ass grabbed, but not being bothered by some one grabbing his shoulder or his neck or whatever. Not polite, but in close quarters, stuff happens. It's not right for either a man or a woman to do that to someone else, and it can bevery shocking.
3, Anything below the belt by a stranger, either a man or a woman, for me, would give me pause. that's an invasion of personal space.
However, unfortunately, people are assholes. They think they can get away with everything, especially in a crowded environment.
Still, that's where you have to either give a dirty look, and say "Do you mind?" or "Excuse me?!"
I can understand not being comfortable with someone of the same sex grabbing them intimately like that, but there's got to be a better way to handle it.
As far as being uncomfortable with attracting both men and women, that's something you have to work on yourself. Perhaps a therapist would be a good idea. That could be a great way to explore this in a safe place. Although sometimes the best way to deal with this is just to lighten up on things; as it appears you still have some issues with self esteem, and being looked at in a sexual manner.
Best of luck; and welcome to adulthood