Homosexual practice however was ALWAYS considered vile and reprehensible, whereas incest wasn't always...and became that way--as genetic defects accumulated in families and tribes--making it a genetic disaster.
Before the Law of Moses, for example (from around 1400 BC) half-brother/sister marriage was practiced by godly Abraham and others, and in pagan cultures, (like Egypt) full incest was practiced (and later, in the Law of Moses, condemned). Of course Adam and Eve's children must of been incestuous--as there was no one else around--BUT, with their pure genetics, at that time it wouldn't of caused genetic problems--those took generations of time to accumulate.
Incest is wrong because of the close family affinity of the couple (eww)....and it is so risky of genetic damage to the offspring, and future generations....
Homosexuality is similarly wrong because of the close sexual affinity of the "couple" (ewww!) and it always results in NO offspring, no future......(the ultimate genetic damage...).
In short the sin of homosexuality is similar to the sin of incest...but, it is even worse.
Any person whose reason is so askew, as to not see the correlation with homosexual acceptance, cannot be debated. Her approach to the issue is to present a straw-man shibboleth, and continue her argument as if braced on the shoulders of Atlas. Ergo, as sound as your argument is, it can never sink into the mental process of a liberal and take root. It is as if sowing seeds upon rock.
Charles Darwin (who is no longer cited for obvious reasons) would say that homosexuality does not propagate the species and therefore does not contribute to its survival.