I think of it from a purely evolutionary point of view. Male-female sex is necessary for the natural propagation of the species in higher animals. Exclusive MM and FF "sex" does not propagate the species, therefore it is not beneficial for our species, not what it needs to continue, and not a preferred trait. I believe it does pop up occasionally in the wild too, but I consider that to be a freak accident just like any mutation.
It may be that propagation is not the only value of sex. It has been said that sex is also a means of "pair bonding" which makes a marriage more permanent. However, a marriage of likes is not what that is about.
Well, if we confined it strictly to the Democrats and RINOs, this country could get straightened (!) out in two generations.