Abnormal literally means a deviation away from the norm. It has nothing to do with morality. I have an abnormal finger on my right hand as it's swerves off to the right, not like most other human beings. That does not infer anything else, example as regards my personality or morality.
In terms of evolution, homosexuality is abnormal, science calls it a paradox, indicating it's not yet fully understood. Homosexuality has always been with us since earliest recorded history. While many theories abound as to why we see it, nobody can explain it. Just because they found a gay gene some years back, does not take away from the abnormality. If you wish to infer anything from the discovery, consider they also discovered a homophobic gene.
While all this is about homosexuality, they have not even really begun to understand lesbianism.
That's while I have all empathy for real L/G/Ts, I have no time for the LGBTQ+ crowd that is also a fan club and other toxic elements mixed in