This web page has as good a description of the problem as I was able to find on the internet:
Most males reach their sexual peak at age 17 or 18, and it's a long, slow slide thereafter, becoming particularly noticeable after the age of 30. It's not uncommon for an 18-year-old male to be able to achieve orgasm four to eight times in a 24-hour period, whereas most 30-year-olds are happy if they manage once.I think this is right on target, because it was after the age of 30 when I really started to notice that something was different. For some odd reason, I remember a morning when I was 24 years old and walking through the campus of my graduate school, and I had a raging hard-on because there were two hot coeds in front of me. But that kind of thing no longer happens.
I still have no problem masturbating, and despite what the above quote says, I can do it more than once in a 24-hour period. But maybe the urge to do it isn't quite as strong as it once was. And that's a good thing because masturbating is a real waste of time and mental energy that could be better spent doing just about anything else.
Just as I still masturbate despite my diminished sex drive, non-virgin men continue to have sex even though they have a diminished sex drive. This is because sex for normal men is a learned habit, and even though the raging horniness of the 18-year-old is gone, the habit of having sex remains.
But for the virginal man who has never had sex, I think that the decreasing sex drive poses a big problem. Without the strong sex drive or a long history of positive associations with sex, the act of exchanging bodily fluids is actually pretty disgusting. I didn't enjoy French kissing Rana. To put it bluntly, sticking my tongue in her mouth was gross.
It doesn't seem gross to all of you reading this because you all have so much experience doing it and thus a whole history of positive associations with it. It's kind of like touching a dead body. Gross to you, but to the medical examiner who deals with them all the time it's not gross, it's just his job. (And then of course there is necrophilia: some twisted people get off on having "sex" with corpses. Yes, it sounds gross, but I'm sure that once you get into the habit of doing it, the act no longer seems gross. Not much more gross than tasting a woman's anus.)
It looks like the problem will only get worse. At the age of 65, a man has hardly any sex drive left at all.
If there are any virginal men in their twenties who are reading this, my advice to you is please, for the love of God, start having sex now before it's too late for you.