How is it fair that gay people may not enjoy otherwise permissible sex?
If by “permissible sex” you mean “sexual activity between members of the opposite sex,” then a “gay" person (person with same-sex attraction) is not forbidden from such activity, on condition that it’s done within the right context, i.e., marriage.
Just because a person has sexual attractions to members of his sex it doesn’t mean he can never get married and have sex. It’s possible for such a person to rightly order his sexual desires to a member of the opposite sex and be married.
But what if a person is never able to rightly order her sexual desires to members of the opposite sex? Is it unfair to say that she can’t engage in sexual activity with members of the same sex? No. It’s not unfair because same-sex sexual activity is immoral—that’s to say, it’s contrary to the good of our human nature.
For something to be unfair, there has to be a violation of some good that belongs to us as human beings. Saying that sexual activity between members of the same sex is something that humans ought not do is not a violation of a good that belongs to us as human beings.
In fact, it seeks the opposite. The moral prohibition of same-sex sexual activity is just precisely because it seeks to preserve the goods of human sexuality: procreation and unitive love. Therefore, to say that same-sex sexual activity is immoral is not an injustice.
For more information on this topic, see my article "Sex, For Goodness' Sake."