I'm no expert on this, but I refer you to Yvain's series on The Last Superstition by Ed Feser: one, two, three, four. As Yvain quotes Feser:

A squirrel will be a better squirrel the more perfectly it participates or instantiates the form of a squirrel. A squirrel who likes to scamper up trees and gather nuts for the winter (or whatever) is going to be a more perfect approximation of the squirrel essence than one which, through habituation or genetic defect, prefers to eat toothpaste spread on Ritz crackers and to lay out "spread eagled" on the freeway. This entails a standard of goodness, and a perfectly objective one. It is not a matter of opinion whether the carefully drawn triangle is a better triangle than the hastily drawn one, nor a matter of opinion whether the toothpaste-eating squirrel is deficient as a squirrel. If a squirrel could be conditioned to eat nothing but toothpaste, it wouldn't follow that this is good for him.

I think the idea is that a homosexual human is a toothpaste-eating squirrel - the instantiation has deviated from its ideal form. And unlike squirrels, humans can reason and choose whether to act in conformity with our supposed nature.

If you assume some unsubstantiated premises, I guess this makes sense. And that's why people are talking past each other when they argue about whether homosexuality is natural. The theist claims homosexuality isn't natural, taking "natural" to mean "conforming to the ideal form". The liberal points to homosexuality in animals, taking natural to mean "appearing in nature".

(I may have horribly distorted this - I haven't read the book.)

Edit: Here's a theist talking about the book, in case you want an explanation from a believer.