That's relevant only if you posit a genetic cause for homosexuality.
No positing going on over here. Just suggesting that male/male and female/female don’t result in a lot of offspring. Zero to be exact. So, it doesn’t matter if there’s a genetic component. There’s no offspring at all to carry or not carry anything. (Although, I don’t believe homosexuality to be genetic, fwiw.)
The Old Order Amish is a very closed society AND they keep excellent records going back centuries detailing various issues with a person (these records have been used by researchers to look into the genetics of a number of medical and mental conditions). Particularly, they have records of why individuals were expelled through shunning.
Such a study could address the following hypotheses:
1. Marriage (including a high degree of inbreeding) and reproduction are essentially required among the Amish. Were homosexual tendencies to be genetic (regardless of whether or not the person acted upon them), you would expect a HIGHER degree of this among the Amish than in the general population.
2. If homosexuals are simply "born that way" the detailed records of punishments and shunnings would demonstrate this.
3. Finally, if homosexuality is simply a choice, and more importantly a choice that is pushed through entertainment and public schools, the Amish can easily demonstrate this. Because they have little or no exposure to modern media, and they are educated in their own schools, and they are subject to a strict moral code, the absence of homosexuals among the Amish would tend to suggest that it is actually either a choice or a mental illness.