I don't know about you but I doubt the legions of gay men and women who have gone through one of the several available routes to conceive children are just "faking it".
You're making the assumption that gender-oriented fidelity and (genewise) fecundity are linked, and they just aren't. A gay female couple for instance might both be completely monogamous and utterly devoid of attraction for males, but in order to fulfill our basic drives humans have the option of deciding to make a cost-benefit judgement and act on it: in this case, that might be artifical insemination or agreeing to engage the services of a surrogate father.
Humans are weird, both evolutionarily and ecologically. Free will and the general craziness of the id will always ensure that artificial rules such as "if (A like X) { !! A+B; }" do not apply.