I was debating whether or not to post in this thread, because I wasn't sure I could accurately convey my opinion on the matter without sounding like the incest defense force. I'll give it a shot though.
Incest is one of those things that's best not talked about. I rarely take this stance on social issues, but there is a close to zero chance that society will ever accept it. Cases between consenting adults are so rare, and so often kept under wraps, that the subject never gets brought up. In this day and age, where people actually openly discuss sex, it's one of the few things that's still considered taboo, and will likely remain taboo. There are good reasons for it, but it being "disgusting" is not a good reason. I find many things disgusting that other people might find beautiful, so who am I to force my opinion on others. From an emotional perspective you can't always pick who you fall in love with, and while I don't believe in "true love," I know that there are situations where the attraction between two people is strong enough to transcend social norms.
All that said, cases of consenting adults in an incestuous relationship should not be flaunted or talked about. It's a private matter best left to the people involved, and no one else. All precautions should be taken to avoid procreation, and marriage should be dealt with carefully or not at all.
Normally this wouldn't even be a subject I'd think about, but I recently read a book dealing with this issue that changed my perspective.
Forbidden by Tabitha Suzuma deals with this exact issue of two "adults" (teenagers, in this case) who are brother and sister and have fallen in love. I'll recommend it on the grounds that it's a beautifully-written novel, but if the idea of sibling incest absolutely repulses you and your mind will never be changed, there's no reason for you to like it. I also won't recommend it if you don't like sad books. I didn't want to like the book, I didn't even want to read it at first. But curiosity won in the end, and I wasn't disappointed. It made me wonder, is it really so wrong?