When males are single they strive to look as good as possible, which results in them (sometimes without knowing it) losing weight, and trying to look fitter. When they are married, they dont give a fuck because their wife is stuck with them anyway so they dont need to look good, the problem with this being that when they dont look as good they dont engage in sex as often, which results in them looking even worse..
Of course, some people actually care about their fitness and stuff (

) so they would keep it up even when they are married / in a relationship.
Women on the other hand tend to try to impress people of the same sex, so they try to look as good as they can regardless of their relationship status, for example emma looks amazing all the time even though she has a boyfreind (i had to to say that..).
Also it is generally more difficult for males to keep lean than for women to stay slim, to the point that most males actually lose the motivation to look good after a while, as they dont recieve tangible results.
IMO, that is a shitty excuse for being lazy, and people should always take care of their bodies... but some people view things differently

it isnt too difficult to go to the gym, or to just workout at home every day/ every other day for 30 minutes, so
most people dont have an excuse
i just found a fuck load of doritos and salsa in the kitchen..

Edit: if someone posts

i will literally throw my cat out the window. Do you want that on you conscience? nope.