Those are sex roles, not gender roles. Gender roles are the socially-constructed stuff, like "men are doctors and women are nurses." Sex roles are the biological, nitty-gritty stuff like "men have penises and women have vaginas."
Right, sort of, in your signature pedantic way. But if it's nitty-gritty you want, then "gender" is only a literary term, not a biological term. Now, maybe you'll agree with this statement: Sperm don't try to fertilize other sperm because nature has provided sex roles for them, in which the role of eggs becomes the necessary counterpart. So then we're back to male-female roles in nature, which have played out quite well for billions of years. I'd hate to see where we would be today if Nature had chosen homosexuality over heterosexuality as way to put her males and females into service.
Sorry for drifting off topic.