BDC, I think you should think of it from the point-of-view of a married man. Of course, if you've never been in a serious relationship for a long time (I mean 10+ years here) it's pretty hard to imagine.
I've been with my gf for 20 years. I love her more than anything. But sexually, there's nothing left. What am I supposed to do? Leave her for a "fling"? Push back my urges and start being miserable and unwillingly make her unhappy because I resent the lack of sex?
I decided not to do any of those. Instead I see a few ladies on the side, a few hundred dollars, no commitment, just sex. When I get home,my urges are satisfied, I'm happy, I don't resent any privation and, I can still be nice to my gf instead of getting mad at her.
Things are never black and white. I used to think like you 15 years ago but now, with more life experience, I can see an sp as a safety valve to stay sane while keeping the love in my relation instead of bitterness. To me, a sp is like a therapist for my sexual urges, same as a chiropractor for my back pain.