Indeed, I think gender (and the way it is intertwined with hetero-normativity) is one of the factors causing us to have less sex than we could have.

For instance, suppose that there is some evolutionary-psychology reason which makes biologically male persons like short sexual encounters higher, and biologically female persons value long-term relationships. (This whole discussion is predicated on the existence of biological-sex based psychological traits, after all, so let's go out on a limb). The expected outcome might be that men seek out men for casual hook-ups and women for longer-term relationships. But this is generally not what we are seeing.

Why not? One explanation might be that most people are not even a little interested in same-sex sex. But that does not seem to be the case, consider for example environments like single-sex prisons.

I would instead argue that the reasons is that "men do not have sex with men" and "women do not have sex with women" is a cornerstone of the socially constructed part of gender, and going against this would necessitating people to drop a big part of their identity, which is unpleasant.

On this view, the existence of gender is actually preventing us from optimally partaking of that obvious good, copious sex.