What do you describe as forbidden?
Anything that irritates me or I see as a taboo or abomination is forbidden to me.
For example, if I have a boyfriend, frolicking around with another guy is forbidden for me. I can have a guy as a friend but it doesn't ever cross that boundary if I have a boyfriend. I can never even have a fling with another guy.
I remember going out with another guy once when I knowingly had a boyfriend. Of course, this new guy knew about my boyfriend (Cause I don't want to do anything forbidden) and my boyfriend also knew I was going out with some new guy just to hang out that day. There were so many times we were in a compromising situation and he would almost kiss me but I would back out. At a time, I had to slap him because he was already turned on and really just wanted to kiss me. I know if it were others, they probably really mind kissing him but to me, that was forbidden. I told my boyfriend about it and he was extremely joyous and confident about me. But then, that's just my "forbidden".