Rachel being surprised that Cosima is gay points to a larger issue of sexual orientation (FYI that is the preferred term not sexual preference) among all humans. Drop the soap and beeble mentioned identical twins and the fact that in some cases twins, while residing in the same womb, might emerge with different orientations i.e. one straight and one gay or lesbian, one straight and one bisexual, or one bisexual and one gay or lesbian.
Maybe the show is trying to say that no matter how much we might want to create multiple versions of ourselves there will always be variations based on how each person's biology is formed while in the womb. The fact that they stated Sarah and Helena are mirrors of each other, Sarah can give birth to the next generation of humans and Cosima is a lesbian points to this fact.
I find this fascinating and am looking forward to the rest of the season. It looks like they are wading into some interesting waters regarding science and religion.