Why are some guys so "dirty"?

some guys can be so dirty/naughty and want to go so far with you. it's sometimes like they pressure you to doing those things that you don't want to. like oral, or anal, or sex. you even tell them that you don't want to but some how you end up doing it while knowing you hate the fact that your doing it. why is that?
Why are some guys so "dirty"?
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