Correlation != Causation tho...
Are the T levels lower because they are married and in a relationship due to natural changes or because they have stopped staying in shape and going to the gym as much since those activities are often done more while trying to improve themselves to find a woman? Lack of sleep due to a new baby? T levels naturally falling as they age? Abandoned their diets to eat out with their girlfriend or wife more, etc? I'd say it would be very hard to make a case definitively one way or the other.
There can be a wide variety of reasons for low T levels, however in general, every generation of male since the 1950s has had lower T levels than the generation before him. So much so they had to lower the "normal range" on the T tests because so many are testing below the original lower bound of the range these days.
Plastics, pesticides, the proliferation of soy and GMO products, piss poor diet with lots of processed foods and refined sugars, sedentary lifestyle, and lack of sleep(a huge T killer most don't realize) are to blame...