Only because the stigma and other consequences fall entirely on women. Ironically, that's the one area where Heinlein was somewhat forward-thinking. But he was writing it very much as a male fantasy: "Wouldn't it be cool if these drop-dead gorgeous women wanted to have sex with everyone, even your wrinkly old ass?"

Note that he doesn't write much about wrinkly, saggy old broads having sex with hot young studs.