Yeah, that "natural behavior" is nothing more than a throwback to a time of superstition and myths. The thing is, all living organisms will always vary, and thus there is no "natural behavior," though there is typical behavior within a species, even each individual organism within that species will vary a lot. That's what makes the gene pool strong. Remove too many variations and you weaken the gene pool to the point of extinction, cheetahs are an excellent example of such a phenomenon caused by a massive population decrease, creating what is commonly called a "genetic bottleneck."
So yeah, your point fails on that tiny fact alone. Secondly, there is no objectivity in morals or ethics, they are decided on by society, and often very flexible. Thus your second point fails on that fact. Third point fails for much the same reason as your first, there is no normal for any living organism, normal only works on machines and physics. Typical is what is common, and there are a lot of typical behaviors humans once had which are no longer even considered civil.
Hate to break it to you, but life cannot abide by arbitrary rules and laws, if it did, we'd all still be single celled organisms instead of stronger colonies of single celled organisms.
Whoa, I wasn't making a point about whether there are indeed natural, normative conventions about biological behavior (or any kind of behavior, for that matter). You'd notice the first sentence of my post contained a disclaimer along that line. My point was that while I am not promoting a "heterosexuality is natural" argument (not in this thread, at least), those that do would object to the specific argument (or "funny retort") as you call it in such-and-such a way.
That said, your arguments against there being a normative ethic seems very problematic. Variations in species does contribute to genetic stability and aid survival, but that does not mean one could extend this fact to arguably ethical issues like human sexual behavior. This is because many people would state that the content of our ethical beliefs has nothing to do with survival. Ethical normativity is something over and above the evolutionary products. You don't have to agree with this, but you can't simply "waive it off" to assume the contrasting position as obvious, either, without giving the arguments their due consideration.
As for your points on there not being any objective moral code or ethical values, I believe we had a thread on this topic just the other day where people argued for and against the concept of an objective morality (here it is). I don't want to rehash that discussion here. All of this seem hardly relevant to the present discussion, anyways.