Last night my bf said to me all girls like rough sex. Knowing that I am the only one his been with I asked how do you know that and he wouldn't reply. Does this mean he has been cheating or something else?
Judging by a lot of your previous posts you don't know that you are the only one he's been with at all. As said he could have got this idea from mates porn even a one liner in a film. You don't know. What is obvious is you seem to be wasting your life getting stressed about a relationship that is stopping you being happy. Get rid.
He would not know how to reply because he doesn't know that ALL girls like rough sex. If you are the only girl that he's been with then he is maybe curious that he is missing out on something, but too embarrassed to admit that he's not that sexually experienced. If his prior experience of sexual relationships has been watching porn then he is going to be a bit disappointed with a normal relationship which from my limited experience is far from all that lip licking and bottom slapping.