I don't know that all male porn stars sleep with both female and male partners. I don't think I can recall when I ever watched a male porn star having any kind of sex with another man. So your first proposition is self-selecting. If you, as a matter of course, watch porn where men have sex with other men, then you might be led to assume that all men (or at least all male porn stars) have sex with other men.
Even if it is true that all male porn stars have sex with other men (and I would have to be convinced of that with some kind of reliable evidence), if I haven't seen it or otherwise recognized that they do so, it would not led me to assume that all men swing both ways.
And, lo and behold, I don't make that assumption...at all.