Here's another problem: The body is not designed to do anything. The body is the product of billions of years of evolution, through the mechanism of natural selection. The body has functions within the limits of these parameters, but design implies a designer, and there's no evidence that this is by design.
If you think gay sex is unnatural because it does not have a reproductive function, then you have limited "natural" sex to reproductive sex, which is a very small piece of the pie. You are also calling into question the "naturalness" of the human body itself, since there are vestiges that have no modern function. Does that make humans unnatural? No, that's absurd.