He probably didn't want a blowjob from you because he is straight. Imagine the situation in terms that would be easier to understand as a gay man. If a gay man was watching straight porn out of curiosity and was maybe even enjoying it, that doesn't mean he would want to mess around with a girl right after. It is essentially the same thing. Also, maybe he also didn't feel comfortable with it because you are his friend? I know that I wouldn't be comfortable with getting a blowjob from many of my friends.