Why would subsequent children get different gene mutations? I don't think it's a thing, it would be like saying that every new child tends to have thicker eyebrows or something.
Of course it is possible that statistically each new child in a family has higher probability to be homosexual for environmental reasons, but for genetic reasons? I don't see it (could be wrong, not an expert at all)
Edit after reading: apparently it's well studied and proven that each subsequent _male_ child has more chance to be homosexual and that "the fraternal birth order effect operates through a biological mechanism during prenatal life, not during childhood or adolescence"
Of course it is possible that statistically each new child in a family has higher probability to be homosexual for environmental reasons, but for genetic reasons? I don't see it (could be wrong, not an expert at all)