This is why the original premise seems so poor to me. Decreased competition? If it exists in both sexes than there isn’t even a decrease in competition. Plus there are so many easier examples. What about people that identify with asexuality? i.e. People that have little to no sexual desire.
Plus, infant and child mortality used to be sky high. People often needed to have multiple sons and multiple daughters.
Humans have been around for hundreds of thousands of years and the entire “sexual orientation” fad began within the past 100 years. Before that, it was commonplace for homosexuals to still be in heterosexual relationships. Heck, even pagan cultures that were permissive to homosexual acts still viewed heterosexual relationships as the ones that were fundamental to society. Homosexual relationships with another man or with an adolescent boy were for companionship and pleasure, but they weren’t the building block of the society and people recognized that.