LeBFG, norms change over lots of things. It doesn't make them comparable.
There are clear links between our understanding of relationships between people, how we socially interact, how we view others and the extension of that into sexual relationships. That is how we decide whether or not it is acceptable for certain people to have sex with certain other people.
How we view certain behaviours and attitudes to objects and how acceptable we deem them to be is a separate sphere, as is the extension of that into sexual behaviour.
It doesn't require specialist knowledge to understand there is a difference between people, object and behaviours. They are all different things.
Some obvious examples...
A couple comes to my house. They are both women. My primary aged child knows they are in a relationship and they act as if they are in a romantic relationship in front of the child. The acceptability of this and the reasoning behind this is to do with how society views which people it is acceptable for relationships to exist between.
A couple come to my house. One of them routinely beats the other because they find it sexually gratifying. My primary aged child is not told of their behaviour. The acceptability of this and the reasoning behind this is to do with what behaviour it is acceptable for relationships to contain.
Certain sexual behaviour is never going to become a norm in society because unless that behaviour is also a norm outside of sex. For example, if a Star Trek enthusiast comes around my house dressed up as Spock, that is fine. The fact that they may also have a sexual fetish about Star Trek uniforms can form part of wider life because Star Trek is part of wider life.
The same rule cannot be applied to categories of people; if somebody is an adult and competent to make choices, then we accept whoever they happen to be in a relationship with. Because we have an ethical obligation to treat human beings with equal respect, regardless of their gender, ethnicity, weight or whatever. We don't have to treat an expression of a desire for violence or power with any respect, although we do have to respect people's right to make choices about things they do in private, even if 'private' means 20 other like minded individuals in a room. Fifty shades of Grey, of course, is not private. It can be bought in Tesco's.