I guess there's a differnece being a porn star vs. being an SP. Many porn starts are married, some to people in the business, some to people outside. I guess the key comes back to is whatever you're comfortable with. I've seen interviews with married PS's and SP's, and the disconnect between having sex and making love seems to be very real for them. Using certain parts of thier bodies to earn their living is just what they do, no different than a carpenter uses his hands or a singer thier voice... the one common thread seemed to be that they reserve some special act only for each other. Something as simple as kissing, or missionary position can delinieate for them the difference between work and home. But in all cases, both were open and honest about what they were doing, and were willing to stop / change if things got too weird.
That being said, there are couple that openly recruit for threesomes, we all have heard about the closet pirate, so there are obviously men who enjoy the fact that their wives are getting well fucked by someone else. And if you combine that with actually making money doing that, bonus.
Could I screw a guys wife while he watched? Not too sure if I'd be comfortable with that, but if she's hot enough and the price is right <grin>. Not watching, but knowing it was happening? No problem at all, as long as the lady is into it, and not being pimped out.
Its all a matter of lifestyle choices, and as long as all partied are informed and consent of their own free will, who are we to judge.