It's an estrogen thing
Alcohol increases estrogen in the body and estrogen is the leading cause of testicular atrophy in human males.
And the more atrophied your testicles are, the lower your testosterone production will be. And less testosterone means less muscles accumulation.
Some self testing on myself proved that for me, a couple of drinks does no real harm.
But if I push the envelope, and secure even a minor hangover, my boys shrivel by about 25 percent or more the following day. A clear sign that estrogen has climbed too high in my system.