I have a friend who once said that “Whatever it is, somewhere, it’s someone’s fetish.” If we accept that as true, than virtually anything can be a sex act. For some people, food is part of sex. That’s fine. But there are people who want to eat human flesh, and they want to do it to satisfy a sexual urge.
I’m trying to come up with some elegant way of saying that if it isn’t illegal as a non-sex act, then it shouldn’t be illegal to do it to get off. And if its only purpose is to be a sex act, then that’s OK too. I can’t quite get my slogan together. I also can’t think of anything that is a sex act and only a sex act (wait, I thought of one-- see below), like missionary position intercourse, that could possibly be objectionable to anyone for some reason other than a strictly moral objection. There are lots of reasons to object to cannibalism, even when the person who proffers the delicacy does so voluntarily, the main one being that we don’t let people traffic in body parts to save lives, so we’re not going to let them do it for lesser purposes, like orgasms; and there’s no good way to guarantee the safety of either party (the donor could have something like hepatitis or a prion disease and not know it, and obviously, the safety of the donor is dicey).
So, my take is that anything that is otherwise legal can be incorporated (pun intended) into a sex act. Things that are illegal in and of themselves cannot be. If you think the underlying law is wrong (eg: dammit, I ought to be able to shoot smack, and have sex), take it up with your legislators.
RE: mentally handicapped and sex. The key is consent. Some mentally handicapped people can consent, and some cannot. I have worked with this population extensively. Some of them live very independent lives, some are still under their parents’ guardianship in their 40s. Some marry, and even raise children. Some have sex expressly to get pregnant. Some don’t want to have children, and use birth control responsibly. Some are irresponsible, but that just makes them a lot like the rest of the population.
Most have normal sexual urges; some, maybe 15% of the population, don’t develop a libido. That’s a lot higher than the proportion of the non-MH population who is asexual, but it still leaves a substantial number who want to have normal sex lives. Either they are independent, in which case, they are able to consent, or they have a guardian, who can give them permission to consent, and that can be blanket permission, or permission within a specific relationship.
If you describe the mentally handicapped as belonging to the population of consenting adults, and then say they cannot consent, this is simply an error. Some of them do belong in the population of consenting adults, and sex for them is legal, and IMO, perfectly fine. Some do not belong in that population, and are taken advantage of. This is a crime. Occasionally, two people who are both legally deemed unable to consent find each other. This may become an occasion where consent is given by guardians for a specific relationship, or it may become an issue where they have to be kept apart. It is a case-by-case sort of thing.
Incest is illegal in some states. It is, now that I think about it, a sex act that is in and of itself illegal in some places. It may not even be states, but jurisdictions. Personally, I think it’s none of my business if the parties involved are adults. Also, FWIW, in-breeding problems arise in isolated populations, where everyone is at least a third cousin. If a brother and sister whose own parents have widely diverse genetic backgrounds conceive a child, that child actually has a pretty good chance of being perfectly normal and healthy. All the same, it’s probably not a bad idea to counsel close relatives not to have children, and marriage incest laws should probably stay as they are. They won’t prevent all incestually conceived children, but they probably keep the numbers lower than if brother/sister & parent/child marriages were legal. But incest in and of itself, between (or even among) adults, should not be illegal.