Is being straight a social construct?

[polyglutton]

Yes, it is a construct!  From a European/Western/Colonial context: it was only in the late 1880s that the concept of homosexuality was invented. Before this, sexual orientation was not medicalised. 
 

Sexologists and anthropologists show that the majority of humans fall within a spectrum of bisexuality, with extremes of exclusive hetero- or homosexuality as a small minority. 
 

A majority of pre-colonised (by Europeans) indigenous cultures worldwide didn't/don't have norms of exclusive heterosexuality. 
 

I highly recommend these books for more on this:

Sexual Hegemony by Christopher Chitty

When Brooklyn Was Queer by Hugh Ryan

Bad Gays by Huw Lemmey and Ben Miller

Dawn of Everything by David Graeber and David Wengrow