As a former attentive health class student, I tend to regard my potential sexual partners in a, "when you sleep with them, you're sleeping with everyone they ever slept with" light. In the case of a porn star, they've probably got some kind of nasty STD to pick up. STD tests aren't 100% reliable, so the odds are too risky for my liking.
Besides, "pornography" literally translates to "a written description or illustration of prostitutes or prostitution." It's a truism that reflects that porn stars are exactly that: prostitutes. From what I've read from people who have had experience with being in a relationship with one before, prostitutes are really lousy mates.
Basically, their world view of men is that they're sugar daddies for whom they can get what they want by sleeping with them. You might think one has an excellent personality, who really cares about you, but you need to bear in mind that fooling men into caring for them is a job skill for which they have had a lot of practice. When they get married, it rarely stops them from continuing this habit from bouncing from john to john.
In other words, porn stars/prostitutes have absolutely zero sense of fidelity - they've completely obliterated it from their world view. You're just another source of income for them, nothing more. You might find an exception, the clique hooker with a heart of gold, but it's well outside the norm.