Actually, if there were only 100 people left on the planet, they wouldn't section themselves off into two groups. If for some reason they were forced to split, (crazy discriminating tectonic plate split) the homosexuals would still produce. They would rewrite the social contract so that people could have loving and/or sexual relationships with the same gender and still also have sex with the opposite gender for the purpose of having children. You forget that sexual desire only exists to promote the more simple desire to reproduce, which is still very strong even without the desire to have sex with the opposite gender.

Not only that, but if the population of the planet is 50-50 hetero/homo then there is an extremely good chance that the population of group B will be 50-50 hetero/homo after 60-70 generations, since after 60-70 years, many of the first generation will still be alive anyway. Also, if it is only group B that has 'genetic malfunctions' (other than homosexuality, of course ) then group A will most definitely dominate in the long run.