Reproduction, or lack thereof could be an issue.
...as it is quite likely that many homosexuals are bi-sexual as well, reproduction, in the natural way, poses no issue for them...and if faced with extermination, any distaste for heterosex will be quickly swept aside, and the act consummated...
...a more useful study would be to determine the population of bisexual individuals, as these people would be more likely to have some lasting influence on the culture going forward, that was unavailable to them in times past...
The Regnerus study looked at adult children raised in same sex households. It found that these kids fared far worse than kids raised by a mother and father, and that they were far more likely to experiment with homosexuality, more likely to become gay.
http://www.frc.org/issuebrief/homosexual-parent-study-summary-of-findings