There is no conclusive evidence that homosexuality is genetic. Twin studies hint at that it is not. I think they came up with the genetic argument to make it more socially acceptable.
My observation (it might be wrong though) is that for people like LeBrok, everything must be genetic unless there are striking racial differences. When there are striking statistical differences between racial groups, then it must be 100% cultural, because claiming otherwise would be racist.
However, he seems to be racist himself against Native Canadians (he claimed that they lag behind due to - quote - "shortage of farmer genes").
I have heard from my Canadian friend, that racism against "First Nations" is still very acceptable for most people in North America. Just like racism against Eastern Europeans (which is why you see LeBrok slandering Eastern Europeans and Native Canadians all the time - these are his two favourite scapegoats, which is kind of funny considering that he is an Eastern European living in a land which formerly belonged to Native Canadians).
And racism against Blacks, Middle Easterners, and some other groups, is of course Non-PC. He is driven first of all by current trends in political correctness and by his "trendy" leftist views, before logic.
My guess is that racism against Natives is more PC probably because they don't fit into the multicultural narrative. Only immigrants of all kinds fit into the multicultural narrative, not aboriginal inhabitants.
All other kinds of POCs increase "diversity" and "multiculturalism", but Natives do not. Natives just happen to be there, and always have been, so they are not making Canada more multicultural than it already is, and this differentiates them from all other groups, making them
"less attractive" for various SJWs etc.
PS:
I was told that accusing someone of racism might be Non-PC if they are leftist or part of another group which enjoys preferential treatment in the West (if they are right wing then it's OK). It is better to call such leftists "nitpicky", "angry" or
"bossy". And that's what LeBrok seems to be.
How is it unpersuasive that exclusive homosexuality didn't exist in the past?
Not only in the past.
Even at present it does not exist in some societies.
For example the San Bushmen and the Pygmies.