I think it's because they've died with their boots on.
Sometimes they lose their mate.
Sometimes it is because of a loss of libido or their body shape slips. Heaven knows that that is not a problem with a lot of swingers nowadays because so many are of the large variety.
We all change as we go through life. I used to golf a LOT! Now I don't. Etc. Etc.
BTW, I'm 50 and have as large a sex drive as ever. I remember taking days off from work because my back hurt, but I'd go and play a softball game that night. Well by 5 I guess. Point is that nobody (the wife) could understand that and nobody really understands this either.
Lot's of reasons here and some actually make sense.
Male D
M.D.