I'm a little bit late to the game, but there are some researchers in Oregon trying to see if they can manipulate the sexual preferences of sheep by tinkering with their physiologies. The moral issues involved have gotten a little muddled, thanks to the combined efforts of PETA and Martina Navratilova, but I think I understand the form of the objections from liberal quarters. In particular, I think Navratilova's big objection is that the implication of working to figure out how to manipulate sexual preference seems to be that we would want to manipulate sexual preference. That would further imply, presumably, that certain sexual preferences are more desirable than, and thus superior to, others.
First, let me say that I think Navratilova mischaracterizes the research being done in Oregon; I think a fairer way of describing things is to say that researchers feel that the ability to manipulate sexual preference is a major test of our knowledge of the mechanisms involved.
Second, I want to just briefly mention one of my pet issues, which is that the distinction between genetics and choice in discussions of sexual preference acquisition is more apparent than real. It just doesn't make sense to talk about people choosing what they like and don't like. When people choose, all they're really doing is taking one course of action instead of the alternatives, but they do so on the basis of their previously-existing preferences. That is, what is a choice supposed to be if it's not an expression of preferences? The most charitable thing you can say about the whole thing is that you can't explain preference acquisition in terms of choice without falling into an infinite regress. Really, though, I think you can file this under "it's not even wrong". Actually, I'll do just that.
Update: To clarify, the only thing, here, I really consider "not even wrong" is the argument that people choose their sexual preferences. That particular position just doesn't make any kind of sense.
The concerns that Navratilova brings up are another matter. Those are coherent beliefs about the issues, they're just wrong or misapplied for various reasons. I don't think that the research should stop, nor am I especially worried about its implications, but I can at least understand what her objections are supposed to be.