The reason this sort of thing still happens is because you don't assert yourself.
If that sort of thing is going on, make sure you say "No" clearly, first and foremost to make sure it's not just a misunderstanding. If that doesn't work, call for help and someone else will back you up.
Never, ever just go along with something you're uncomfortable with. If you do, you enable the bad behavior.
You should consider learning some things about self-defense as well, so you know how to protect yourself in case someone gets handsy and you can't get help. As a rule of thumb, fighting isn't about strength as much as it is about brutality. If you smash someone's nose into paste with your skull, they will let go basically every time, doesn't matter how strong they are. Learn your options, find someone you can practice with--a strong male friend with a background in martial arts can go a long way toward getting you into shape for self-defense, since you'll be practicing with someone stronger than you--and you'll be in business.
Remember, ultimately your safety is your own responsibility. While people in general will want to help, that may not always be an option, and you can't rely on the good graces of predatory people. Be ready to defend yourself, and be mentally prepared to seriously hurt someone, if you have to, in the process. You can't take your safety lightly.