If Incest is bad because of biological defects, why is homosexual incest still bad? Why do YOU personally have a problem with it?

I'm more of a moral objectivist. That morality is driven from whatever causes the least suffering to other people. Whether we can successfully identify moral behavior with information at hand is a different matter.

In this case, the birth defect risk of incest could harm a child born from it. Thus the moral quandary.

Homosexual sex, producing no children, is thus not immoral as there is none to suffer the choice between consenting adults.

One could argue for equality of standards across different types of sexual groupings based on the notion that if incest was normalized for homosexuals, society would in turn feel more accommodating to heterosexual incest and thus make it more prevalent.