CFM: Wait. Imoen? Ugh! What is this, Game of Thrones? lol
StuffyouonaMatress: This isn't an ethical breach of human rights. What is morally wrong about romancing Imoen? Two independent adults who can give consent, let us have fun under the sun.
It's probably because a lot of people see Imoen as your "sister", both from an upbringing point of view and the fact that... Wait, I'm not sure how far you've played through the series so I'd best not spoil anything for you. XD
Regardless, there IS an Imoen Romance mod out there. The latest version is actually very well written and has many numerous branching paths and possible endings you can get. For example, you can romance Imoen, romance her BADLY, just be friends with her (in which case the mod essentially functions like an "Imoen dialogue" mod that makes her more talkative and gives her more comments on things you encounter), help her find romance with someone else etc. There are even different endings for all of the above depending on your alignment and how you treated Imoen during the course of your adventures.
I wasn't around at the time, but apparently the early versions of the mod were... fairly disquieting. However, it seems the latest versions were done by a different writer who's kept the character of Imoen much more in line with how she's written by Bioware.