Interestingly, former porn star turned comedienne Alia Janine has said as much in an insightful article for Laughspin. In it she lists five aspects that make the two professions similar, one of them being that they both attract people with "issues".
Then there’s the personal demons that stem from our childhood experiences like alcohol and drug abuse, depression, overactive sex drives, abusive tendencies, and on and on. We all have issues, and we all have skeletons in our closet. How we deal with those issues is part of what makes us who we are. Comics and pornstars tend to have very similar childhoods, but comics choose to use comedy to express themselves and pornstars choose sex. Neither is the right or wrong way; it’s just how that person is wired. And then there’s also many comics and pornstars that have had “normal” childhoods, but still chose their professions. The two groups just tend to click; they get each other.
I think there's a lot of truth to these observations ... She also makes the valid point that both genres are low on the entertainment food chain. It's almost guaranteed that you're never going to become movie star rich in either of them.
Related to this aspect is how they are perceived by people generally. They are seen as second rate genres in their respective spheres. Comedy, by definition, isn't to be taken seriously -- unlike drama. So a brilliant standup comic will never achieve much respect as an artist unless he branches out into acting and delivers an emotionally deep and complex performance in a well known film or theatre production.
Needless to say, porn actors are laughed at and looked down on. They are seen as the gold standard for excruciating incompetence in this craft.
There's one other similarity that comes to mind. That is that they are both one dimensional genres compared to other forms of entertainment. Standup is all about getting laughs. Everything is subordinate to that goal. And with porn it's about pretty much nothing other than giving the viewer an orgasm.