From a sociological perspective, we need to have reproduction to ensure our survival.
Not only is our survival not in peril, our proliferation is making a bollocks of the planet not only for ourselves but every other species on it. With a human population still undergoing an unstoppable rise any argument predicated on the survival of the species, as though we were still a gaggle of hairy hominids on the savannah undergoing a disastrous population bottleneck, is a bad joke. That scenario occurred at least once - it's thought that at one point around 70,000 years ago the human population crashed to anywhere between 10,000 and 2,000 individuals - but extinction through low numbers is now really not the issue.
Homosexuality in animals can be described as natural but actually it is unnatural because it is contrary to how the body is designed to work.
Except that it wasn't designed. Form isn't always function and you still can't get that ought (or, as is invariably the case, ought not) from an is.
Sex in an infertile couple is natural because it is a natural act, being the same act that brings about procreation.
Unless you don't understand the concept of infertility, that is an act from which no procreation will result.
It also unifies a man and a woman, which is good for society because it is consistent with the structure of the family unit.
Now we're getting somewhere - possibly. This is exactly and precisely what I've already said - sex is a bonding exercise between two people (or more, if you're lucky). Except that rather than your narrow parameters, it's also a bonding exercise between people who don't have a family, don't want a family, can't have a family, gay couples, etc.
Sex in homosexuals is unnatural because it is an act that can never bring about procreation.
And this is precisely the spot where your would-be argument collapses, since you draw an entirely artificial and
ad hoc distinction between two scenarios with precisely the same effect or outcome, or rather the lack of one. Heterosexual sex between two people who can't reproduce (one or both parties are infertile) gets merrily waved through, but homosexual sex - with precisely the same result or non-result - is impermissible, according to you. This glaring inconsistency can only stem from animus against homosexuality, which as we know is mostly religiously inspired.
It is bad for society because it is contrary to the structure of the family.
I don't fetishise families as so many seem to, for one thing. There are a billion and one ways of living, enough to suit all kinds of people in all their dizzying variety, of which a family is only one kind. It suits some, but it doesn't suit others.
As for society, my conception of a happy as well as fair and just society is one that extends to its citizens the greatest possible personal freedom and the maturity to treat them as competent actors capable of directing the course of their own lives and finding their own happiness in their own way according to their lights. People like Hope call this rampant individualism; correct. It is. The sort of social cohesion that he has talked about may have some good points, but the bad outweighs the good; it has a tendency to squash individual freedom and to enforce at best a bland and invariably hypocritical conformity and at worst oppression, overt (such as we see in strongly religious societies - Saudi Arabia's morality police, for example) or covert, through what John Stuart Mill called 'the tyranny of the majority.' In passing I would add that in
On Liberty, as near to a Bible as I've ever had (or one of them) Mill talks at length about how strong, mature, confident societies have to cast off that sort of explicit or implicit tyranny and not merely tolerate but positively encourage diversity and dissent.
