Maybe gays and lesbians are like worker bees. They perform some function in our society that helps ensure our survival despite not actually reproducing? Well, I know gay guys are really good with style 
All joking aside. I can KINDA see where this guy is coming from, but I just feel like he’s trying to take a swing at gays and although he may want to give them rights, would like it to be known that they aren’t right. But anyways, I’ll try to entertain the sane part of the question.
The purpose of basically any species is the perpetuation of itself. We are all essentially competing designs that reproduce and improve themselves. So, he’s saying that the point of humans, like all species is to reproduce, and since gays aren’t a part of it (although we’ll leave out lesbians which CAN reproduce) then they are abnormal.
But you have to then realize that Homosexuality isn’t solely a human trait. It’s been found in animals as well. Following the logic that our jobs are to reproduce, then birth control, and any form of sex that doesn’t result in a realistic chance of pregnancy is abnormal. In this regard, if you are going to punish or castigate homosexuals, you have to castigate those who are infertile (not their choice, like homosexuality). And if you want to look further in the future, would it be okay to allow men to get pregnant (like in that awful Arnold Swartzenneger in Mr. Mom?!?!) The point is that the OP is saying that homosexuality is something that occurs in a small percentage of the population. True, but that doesn’t make it wrong. It similarly isn’t wrong to be black, short, tall, or stupid or smart. We’re all different. Homosexual men can reproduce with the help of artificial insemination and a surrogate mother.
I also agree with the idea that something that is inborn can’t be considered unnatural. Not every single aspect of a species is geared towards successful mating. Dolphins mate for fun. Monkeys have been known to masturbate. There’s a pair of gay penguins in New York City!
The logic he’s used is this:
The purpose of a species is to reproduce
Humans are a species
Gays do not reproduce
Therefore gays have no purpose in the species.
However, it has to be said that doing something that doesn’t contribute to reproduction doesnt’ necessarily mean it’s a bad thing. Gay people do plenty of things that indirectly contribute to reproduction. Let’s say that in your hypothetical world of 4 men and 4 women. If one of the men is gay, he can certainly help by farming, in order to feed the others. Anything he does to help, helps in some way. Or let’s take it a step further and say there’s a lesbian in the mix too. Would it be good to have all four women pregnant at once and hardly able to forage for food? You’ve got three pregnant woman and one to help them as well as the four men. Maybe it isn’t the optimum for maximal reproduction, but these people have to survive to an age where they can reproduce too. You could hypothetically construe a world where having a certain population that wouldn’t reproduce as a good thing to limit growth to sustainable levels. The optimum way to do it would be to have a ten woman to one man ratio, or even higher, but that’s not how it works, is it? The goal isn’t to have the fastest level of reproduction.