>>5
Assuming haploid gametes and diploid zygotes, one could imagine a situation in which there is only one kind of reproductive organ/gamete. Upon mating, both unisex organisms receive the other's gametes. However, the gametes themselves contain some sort of MHC-like molecule that will distinguish self-gametes from non-self gametes. Thus, two-self gametes could not come together in the womb to form a zygote, whereas two non-self zygotes could.