Metaphysically, i.e. just holding forth, the position of those who held "quite liberal position on sexes" must seem less preferable, taken for granted the famous Occam's Blade method.
A) By the Occam's Razor we should avoid to plural many over-theories, and to prefer T.1 to T.2 iff T.1 < T.2 ('<' means 'simpler', or 'has less above suppositions'). B) Also, it's not correct to think that naturally we should take T.1. instead T.2., because of our current state of living. C) The Nature's been corrected our ways of living by circumstances. So, our position - just like that - is circumstantial. D) Relying on what must be more helpful for us is correct as long as it's rationalized. E) The Occam's Razor principle is very rational. F) If E we must prefer A to B. G) Till now the Nature's been providing much more examples of two sexes and not gay behaviour (even in the simplest creatures, e.g. infusorians, circled words, jellyfishes, and so on). H) Taking F for granted plus being observed with G we must prefer two sexes and not gay as more rational and natural as well.
I don't think such arguments above to be sound, but I think the subject might be prolonged and detailed more. I lend your support thoughts here.