Am I doing something wrong? In relationships?

There's nothing wrong with you if you're getting cheated on. Its the guys who are at fault. You deserve better than them. DON'T give into any of them if they wanna go further than kissing. Just don't. If that's why they cheat on you, because you won't sleep with them, they're superficial losers who will end up high school drop outs with a pregnant girlfriend who they'll marry and divorce working at the tire shop down the street. Just sayin.