Natural law theory is supposed to be based on the observation of nature. When it comes to homosexuality, the church stopped observing nature about the time Thomas Aquinas died. If it had been more observant the church would have noticed that homosexuality is extant in all species of the animal kingdom.  As for reason, if Holy Mother Church had observed nature a little more carefully she would have used her capacity for reason to arrive at the very reasonable conclusion that for a small segment of the human population homosexuality is a natural variant and therefore not in and of itself a sin.

Addressing your earlier comment, allow me to say that gay people are just as capable of sinning sexually as straight people, even those within lawful marriages, when we objectify our sexual partners for our own satisfaction at the expense of our partner's personhood. Your unspoken thesis suggests that gay people are asking for special exemption. We're not. We're only asking to be treated equally.