I think that if it were as simple as that, far more girls would be lesbians. I'm not saying it can't be a contributing factor in an "nurture" way but I severely doubt it is the sole cause of someone's orientation. I'm sure a lot of straight girls were very close with their mothers growing up and are perfectly hetero. I did have a therapist once who didn't believe anything non-hetero existed, even going so far as to tell me "lesbians are just looking for a replacement for their mother's love" but I discount her opinion because she was a bigot.

I can see how a female-orientated comfort system might cause someone to prefer female company in friends or close bonds in adulthood but I'm not sure how it might cause someone to experience sexual attraction to females instead of males. It seems like a rather drastic cause and effect to me.

I think where causes of orientation is concerned, one can think up a hundred and one possible triggers but we may not know for a long time what actually does affect us. For any one idea, there may be a hundred cases who contradict the notion. In any case, as Indie said, I personally think acceptance is far more important than conclusive understanding.