Elephants never give birth to twins and have extremely long pregnancies. Frogs fertilize eggs by the thousands. There is no reason whatever to assume that ALL members of any given species need to achieve maximum efficiency at reproduction.
There are plenty of animals that seem to exhibit "gay" behavior. there is no practical reason for Fido to hum your knee, and yet, he does.
Do you think he does it because he is sinful, possessed by demons, or it is just something some dogs do by instinct?
If it is the former, then you are thinking like a medieval monk. If it is the latter, then it is simply natural behavior, and it is obvious that the desire to have recreational sex with no possibility of reproduction is simply a part of the make up of canines. So why is is not logical that humans not engage in the same behavior?
Why assume that reproduction is the ONLY reason for sex?
Eating is required for survival, but humans eat all sorts of things that are not ideally nutritious, just because they taste good.
Most people can enjoy the sweet taste of a diet soft drink, even though it is not nutritious and may even be harmful.
Suppose someone says, "You should not drink Diet Dr Pepper because it has no nutrition in it". And then DENY that people could possibly be born with any built in reason to like the taste of artificially sweetened soft drinks.
Some people like really spicy foods, some cannot stand them.
Saying that gay sex is bad because you have no desire to have gay sex and it seems unnatural to you is like saying that artificially sweetened or highly spiced foods are bad because you do not like them and consider them to be unnatural.
Diet soft drinks, by the way, are by nature unnatural. They are sweetened with chemicals that are not found in living organisms.
For tens of thousands of years, humans never had an artificially sweetened food product. During the same period, people have had gay sex.
I suggest to you that a Diet Dr Pepper is far more unnatural than Adam having sex with Steve.
It is not like anyone is forcing YOU to have gay sex.
It is far more likely that someone will try to lure you into drinking a Diet Dr Pepper than having gay sex with you.