It's not about ability. If a queer person "sticks to their guns" in terms of sexual attraction, they won't procreate. Hence, nonfunctional sexual orientations. Even you say that in order to procreate there is a need for a healthy egg and a healthy sperm. And those are tied to XX and XY chromosome configurations respectively, while sexual attraction is merely a subjective perception of our innate reproduction instinct. Call it "God's mandate" or simple biology, the net effect is the same as that’s how this universe obviously rolls.