Firstly, FGM is illegal and MGM isn't.

Secondly, FGM is rarely if ever homologous to MGM - it's usually much much worse. Only Type 1a is the removal of the clitoral hood, and that is homologous to the foreskin (and type 1a often involves removing the labia) - see classifications. It's like comparing cutting the foreskin to cutting out parts of the rest of the penis). FGM often involves cutting out the labia, sewing up the sexual organs. It's not the same - you're comparing something bad to something godawful.

Conflating FGM and MGM makes it harder to combat FGM. People who would otherwise be against FGM might change their minds 'because it's just like circumcision and I didn't mind being circumcised'. They don't realize how bad FGM is. It also makes legislating/taking action against FGM harder because all the people who want MGM legal will fight against it.

That said, MGM is far more common and accepted, and that should be combated, but not in the same breathe.