I can think of three plausible reasons for this:
(1)
The couples are together so much, and have so much love between them, that their bodies somehow create a kind of equilibrium and endrocrinal activity disrupts from standared evolutionary homeostasis to instead form a new system.
(2)
Instinctually men while married are no longer on the hunt or need to be terratirial / threatened by other men in competition for a mate. They have found a settlement and are comfortable. Hence there bodies over time adapt to this new confort zone.
(3)
Marriage is a slow dynamic process that turns ordinary manly men into feminine pussies by lowering their man juice. Also, all women are secretly aliens from Venus that originally came to earth in a large elaborate scheme to harvest all the precious Testosterone they could from us in order to feed Jizzabah, their mysterious and ultimately evil spider god.
I personally believe in option three, its the only logical explanation if you really stop and think about it.