Incest, homosexuality

[ib.me.ub]

But why should it not be allowed?

Inbreeding (reproduction between two related individuals) may also involve incest. Inbreeding leads to a higher proportion of congenital birth defects through an increase in the frequency of homozygotes.[49] The effects of this can diverge - recessive genes that produce birth defects could become more frequent, resulting in a higher rate potential of these defects while genes that do not code for birth defects can become increased within a population. The overall consequences of this divergence depends in part on the size of the population. In small populations, if children born with heritable birth defects die before they reproduce the ultimate effect of inbreeding will be to decrease the frequency of defective genes in the population with an overall decrease in the number of birth defect-causing genes over time. In larger populations it is more likely that large numbers of carriers will survive and mate, leading to more constant rates of birth defects.[50] A 1994 study found a mean excess mortality with inbreeding at the first cousin level of 4.4%.[51] At any rate,the degenerative effects of inbreeding will only be significantly fatal after two or more repeated incest cases, and that varies depending on the number and quality of inherited congenital disorders of which the family members may be carriers. A study of a group of 21 made up of brother-sister or father-daughter offspring found that 12 had abnormalities with 9 of which were classed as severe.[52]

I know you said without having children, but who is to control this aspect of the topic?

Edit: Nature is obvioulsy trying to tell us something here.