From an evolutionary perspective, no body parts were designed with a given function in mind. We came about as a result of random mutations. Those that had mutations that made them more likely to reproduce were more likely to pass on their genes. None of our body parts were designed for any particular purpose. And there are plenty of cases in evolution where one thing that was originally beneficial in one way later evolved so that it was used in some other way (such as the bacterial flagellum). You are falsely attributing purpose to a process that is not driven by purpose. Now you may disagree with this account of how we evolved from a theological perspective, but that’s essentially the secular view of evolution.
If it changes anything, let’s say that it’s possible that humans have evolved to have a separate reproductive system and a separate gastrointestional system. Whether or not evolution occured or to what extent, such evolution has dictated that the two systems be completely separate, not dual function like reptiles and birds (at least at the terminal end). Seems you have helped advance the argument that evolution does not favor gay sex. Thank you for proving my point from an evolutionist perspective.
Again, evolutionarily speaking, there was no design. But aside from that, there are plenty of ways we use our bodies differently than what they were “designed” to do. For example, the human eyes were not “designed” to read from a computer screen (and there can be serious problems for those who use a computer a lot). But I don’t think that’s a good reason to say that it is wrong to use a computer.
Again, you prove my point. Doing something that we know is harmful; looking at a computer screen “a lot” damages the eyes. So we shouldn’t look at computer screens “a lot” if we don’t want to adversely affect our eyes. Gay sex is harmful to certain parts of the anatomy. It follows that we humans shouldn’t participate in gay sex if we want to preserve the normal function of those parts of our anatomy.
That is my point in responding in part to the OP. Thanks for supporting my argument.