“We now treat the differences between male and female as socially constructed and those between heterosexuality and homosexuality as innate and genetic”

Interesting. Can we do that? I thought about it a little bit. And ya, ya we can.
Here’s why.

gender roles, socially constructed or not, put a person into a narrow category. thus, if gender is socially constructed, we are socially forcing people of different personal persuasions into the same narrow column, despite a persons’ biology. if we acknowledge homosexuality, socially constructed or not, we are allowing people into a spectrum of categories. So to say that gender is a socially constructed norm is actually analogous to saying sexuality is a biologically constructed varient, because both acknowledge that people are simply born the way they are, variation from the norm and all.