As you hint at up above, there are non-religious people who believe regulating sexual behavior is important for society. Do you have a way to counter that idea when it's based on a secular worldview which is not so alien to yours?
One reason might be that they believe that having a culture which is hedonistic toward sex is harmful for society, and they believe that acceptance of homosexual behavior necessarily leads to having such a culture.
>presumably Stalinists?
That is definitely not a reasonable assumption to make.
>but if you're not religious and don't believe in utilitarianism, what exactly is the goal for your society?
It is perfectly possible to be utilitarian, non-religious, and opposed to the acceptance of homosexuality. All it takes is a belief that acceptance of homosexuality is on the whole and over time more harmful for people than intolerance of homosexuality.