Unless you're going to argue that over 50% are secretly gay, then it still has a significant biological factor.
Similar to left handedness (or right handedness) that's totally a concious choise... 2 left handed parents still only got less than 50% of their children being left handed, far from 100%, but much higher than the population average. Having one laft handed and one right handed identical twin is not that uncommon, way more common that having 2 left handed one for example.
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