There seems to be a lack of interest to answering this question. Hm.
Is this the same thing as explaining a joke. If we explain why, will it stop being fun?
Here's my ad hoc list of possible reasons, just for the hell of it.
1) Rejuvenation
2) One possible response to the never ending stream of youthful feminine images in the media.
3) Incongruity of the significant other in comparison to point 2.
4) Lack of response on the home front.
5) Lack of a home front.
5) Cuz you can.
6) An inexplicable enchantment with the female form at its best.
7) The incomparable sensation of a woman's skin ...
8) ... especially when experienced by the entirety of one's own skin.
9) Cuz you're not supposed to.
10) Cuz you might go insane if you don't
11) Cuz the smile on your face needs maintenance.
12) Cuz the liklihood of addressing any of the above outside of a paid arrangement has a probability value near zero.
13) Cuz the liklihood of addressing any of the above outside of a paid arrangement, without incurring divorce court costs, has a probability value near zero.
14) Cuz the liklihood of addressing any of the above outside of a paid arrangement, without incurring relationship baggage, has a probability value near zero.
15) Cuz there is some measure of control.
16) Cuz we don't know that if we stopped paying, they'd give it away for free! (OK. that one's dubious).
17) Cuz evolution demands countless attempts at procreation no matter how fruitless
18) Cuz nothing really matters anyway so why the f*** not.
Why men philander is a question worth pondering seriously. Hits at the heart of one's humanity. And womanhood for that matter.
Point 2 I find irksome. What the heck is it about the image of a young woman that is so innately captivating? And why only those of a particular form, e.g. slender or whatever turns you on?
Why is a sunset beautiful? Why do we enjoy music? What IS music?
Maybe the living of it is better than the pondering of it, with apologies to Socrates. "The unexamined life is not worth living." Or better yet, "The unlived life is not worth examining."
Regards