Just as a fair warning, my comments are not Imoen-romance specific...
We almost might as well also disallow relationships between elves and humans. Because we consider it gross, not right, or at the least, a doomed pairing in life when people are in relationships with someone much older than them. Think of how we'd feel if our friend/family member was dating someone hundreds of years older than them? I must admit that the idea of a X-hundred year old elf getting with a 20-30 yr old human always seemed a bit off to me, and would/should be a rare occurance if it happens at all. But it doesn't really bother me, I accept it because it's just a game after all. On another note, incest is explored in many instances in fiction. I myself have read many acclaimed books that incorporated incest. It's an issue that, when explored, opens up a lot of avenues for exploration of societies and the human psyche. If anything, the use of incest i've seen in fiction discourages incest by showing its possible consequences and impact on people. Along those lines of thinking, I don't see a problem with exploring the issue of incest in fiction (which BG is, after all). Now, I wouldn't take BG as seriously as I would take a novel that attempts to make social commentary on incest, which is why its use in a mod incites feeling in me even less.
I'd also be interested to know how the Imoen-Romance mod is affecting you, your life, or the lives of the people around you in such a way that you feel the need to openly object to it? Is it just because someone asked your opinion and you're giving it to them? Or do you fear it will negatively affect others and you want to prevent that?