I believe there are several modeling agencies/studios around the world that specialize in researching and promoting look-alike models for the porn industry.
Over the years of viewing erotic material of all kinds, I have spotted various guys who seem to have been famous celebs, but on closer examination were look-alikes. Sure, it's a great fantasy to imagine your heart throb star modelling nude and doing naughty stuff, but it's more likely that the hot hunk you see in the porn pic is a model, not the real celeb. Of course there are some stars who did nude shots early on in their budding careers, the trick is to find the real photos not photoshopped recreations.
Aside from porn, there are many agencies who have models on call as body and face doubles (stand-ins) for major screen stars. I've seen pics of lookalikes for Clint Eastwood, James Caan, Robert DeNiro (when young) just to name a few, that fooled lots of folk.