It is easier to say what is not natural, but I will try to explain what I believe nature is.

First, the fact that people, or animals, engage in an act does not make it natural.

An act that causes progression in mind, spirit, body, or race is natural. One that causes stagnation or regression is unnatural.

This is a little vague, but it is the best I can do.

Homosexuallity does not progress the mind, or body. It causes a stagnation in the race as reproduction is not achieved. It also causes a degression in the spirit. It cannot be natural.

Sex, when properly engaged in, progresses all four. It is natural.