They can't reproduce. In Fo3 you learn that when the first super mutants were made they were normal people who over timebegan to turn into Mutants. If you read all the terminlas you find ou that the test subjects began to lose their male/female parts and they grew more and more alike and unhuman. If they have no reprductive parts they can't reproduce
Yeah, FO3 changed that. Don't remember if it mentioned losing their reproductive organ, just their male and female features, but Marcus says he can use his, you know. You can "throw him a party" at the Cat's Paw Brothel, so...