Things like necrophilia, coprophilia (scat), pedophilia, and beastiality all carry an ick factor for me. The main factors for this are consent issues and health issues. There is a world of difference between anal sex or analingus and coprophilia.
As for things like homosexuality, anal sex, BDSM, etc, I do think that changes in access to these things and in how we understand them has changed attitudes towards them. I mentioned something about this in the anal sex thread here in Tilted Sexuality. Three or 4 years ago, I don't think I would have ever considered the idea of analingus, but exposure to the subject eventually changed my perception of it.
The same goes for things like non-traditional relationships. I can't say I was ever *opposed* to the ideas of swinging or polyamory, and I'm sure I would have always responded to the idea of swinging with a "heh, cooool" or somesuch, but I'm much more honestly open to the ideas now than I have been in the past. In fact, one could say that I'm quite interested in the idea of a semi-polyamorous relationship now. Not really interested in anything like a 4-way marriage - I value my one-on-one relationship with onodrim too much for that - but I actually have a significant interest in something at least at a level similar to ratbastid/lurkette and their "umfriends"
Even in this area, my attitudes have changed. When I first started being more seriously interested in the idea of alternative relationship lifestyles, I definitely leaned more towards the idea of casual swinging/sex. After being able to access more information on the subject and, through TFP, be in contact with people who are in various types of alternative relationships, I know now that I would prefer and am very interested in the idea of a more meaningful experience which goes beyond casual sex to a profound friendship - a "couple" of couples, if you will - moreso than I am interested in the idea of casual and uncommitted swinging. Whether the opportunity will ever come up, I couldn't tell you, but I know that my access to information through the internet has had a significant impact in how I think about these things because of the way it has allowed me to interact with people who have experience in them.
As far as UsTwo's point about the eventual legalization of homosexual marriage eventually leading to the legalization of multiple partner marriages and so forth, that's the logical conclusion, but things don't always follow logic. I don't think legalized multiple partner marriages are all that close, but that's not to say they may not eventually happen.