So basically saying that something is a social construct isn't to say it is any less "real" or valid. In a somewhat awful piece of circularity, we can say that they are real because we treat them like they are.

"They are socially constructed labels approximating underlying sexual identities. Sure they could be broken down further for accuracy, the spectrum is real not a social construct."

So again, when we say that sexual identity is a social construct, we aren't saying that there is no biological referent. Being male has a clear biological referent (duh) but what constitutes a masculine identity is a social determination (likewise with sexuality and race).

And again, we're not saying that sexual identity is "learned" or "innate", we're saying that the very idea of having a sexual identity is a social construct. The fact that we are using labels like "gay" or "straight" or "black" or "white" or whatever is to say that we are defining these concepts socially in ways that have no necessary grounding in objective truth.

A lot of what you're saying is just "this seems absurd" and "I disagree", which is of course your right to say but not really a productive line of discussion. Try to abandon these preconceptions if you can and read into the subject a bit when you have the time. In the meantime, as a thought experiment, try replacing "gay" or "straight" with "noble" and "peasant". The fact that these are obviously social determinations doesn't make them any less real as identities for the people who are living with them; they may also have clear biological referents (e.g., being born with certain family traits or from a certian line) just as there might be clear biological markers for homosexuality. But they are nevertheless social determinations about what constitutes these (in the most reductive sense arbitrary) classes, not objective facts.

I don't think all experts on sexuality think it is all socially constructed

So this is kind of the crux of the misunderstanding. It isn't a question of whether an individual's sexuality is the product of the culture or not (though obviously culture and biology play are involved, just like in just about every other aspect of human development). The very idea of "sexuality" as a label that we can apply is a social construct (as are gender, race, and class). This is not to say that these identities are invalid or unimportant, merely that they are aspects of society qua cultural unit, and that as social determinations they vary across cultures and time periods rather than being descriptions of objective truths.