We are setting up Trans World in the state boards this will take a few days Perth has already been done.these boards will be open to general public and they can also comment without membership. This is a trial only.
It is true that some young girls begin having relationships with men at that time. So while they may believe that having sex is the reason for the changes, it's really just a coincidence.
My perception would be that regular sex burns calories. So if burning more calories than replacing, then weight loss would reveal muscle tone.
It could then be perceived as gaining size as less tissue is covering muscle.
I doubt though it would build muscle unless repetition under muscle load is frequent enough to build muscle.
Some BDSM activity could build muscle tone I would think.