I have no beef with that. When I read Mac's original post, I immediately knew what he meant. I didn't see any specific need to go on about it for over 20 pages..
In light of this, if one's equates homosexuality to whether one believes it to be innate, one might say in opposition that, homosexuality is not natural, that it is a manifest result of several environmental influences, that, if not otherwise present, would not cause homosexuality to occur. In that, homosexuality would not occur without interference. In this sense, assuming one ascribes to the notion that homosexuality is not innate, the context and usage of the word unnatural is appropriate.
Tim-