I can see several issues. There's the risk of infection, rubbing cocks with other dudes, being last man in for a creampie, having a permanent record on store shelves and the internet... but the real show stopper for me personally:

It would mean having to give up on the prospect of having any sort of long-term romantic relationship at all.

I know that one solution is having relationships with other pornstars, but a fair exchange would make no difference to me - I still couldn't handle my girl being fucked by other men. I suppose there is a slim chance of finding a non-porn girl who'd be happy to stay faithful while I go off and fuck whoever, but I'm not even sure how I'd feel about that... if she really loves me, maybe she should mind.

Of course, if the opportunity ever presented itself, my other brain would probably take over and choose for me... but the one in my head sees the downside.