Super mutants are the results of vault 87's FEV (Forced Evolutionary Virus) tests, fev forces whatever organic material it toutches to evolve, mostly over a period of days or weeks. Super mutants are humans who have been exposed to fev and have become a super mutant themselves.

Regardless of previous gender, fev likes to start off with a "clean slate" and wipe away most genetic code within the host body, for humans, it wipes away gender, and genetic code, anything that makes a human unique, it keeps doing it until the host is almost unrecognizable as a human, vaugely in the shape of one, the "blank slate"

It then goes about making changes about the body, most notably (and most obvious) is the increase in muscle mass, giving the host a very male look (even if the host was female originally) but supermutants lore wise dont have any genitalia and it is unknown if they can naturally reproduce, the only known way super mutants multiply is by capturing human prey and exposing them to fev,