Yes, homosexuality was previously classified as a mental disorder due to homophobia, but the point is that we now recognize that was WEONG and it is no longer classified that way. Psychologists also previously thought black people and women were cognitively less intelligent and needed to be either enslaved or managed by their husbands; we now know how ridiculous and prejudiced that was! Since I am a professor of gender & sexuality studies, I highly recommend that you read Anne Fausto-Sterling’s famous book “Sexing the Body” which explains why the belief in evolutionary psychology you described (that humans are supposedly pre-wired to be straight for procreation) is incorrect and how that belief developed out of misguided homophobia and a misunderstanding of both biology and nature. It’s a really important book all students should read!