A Comm for Historymemes

I would say that the offensiveness is more in the dubiousness of the specific claim.

Environment influences all aspects of the human condition, so it would be unreasonable to completely discount changing environmental standards.

However, considering the nature of the oppressiveness of past polities and the emphasis on conformity, and especially in Western and Islamic countries which were dominated by Abrahamic faiths which put a high moral value on sexual conformity, it seems premature to want to assert that there is a biological reason for the increase in visibility of LGBTQ+ folk in the modern day. The combination of the extreme individualism and iconoclasm of liberal society, the departure from faith as the cornerstone of morality, and greatly improved recording practices seem much more likely culprits.

For example, in the society of the Roman Empire before Christianity's rise, wherein conformity was highly valued but sexual behavior was not core in moral valuations, only some ~25% of the first 200 years' worth of Emperors were heterosexual, as we would recognize it. There are an immense number of comparisons of a similar type in societies we have records of.

And, for that matter, iron deficiency was much more widespread in the past than in the modern day, quite provably, both by direct evidence and inference from diet. So that particular route is especially improbable.