My answer is not going to be in any way complete, but just to add ...
Sin can enter sex between a married couple because of what happens in the mind. If either partner is fantasizing about someone else, then essentially they are committing adultery even though with their spouse.
If either partner inflicts their desires on their spouse in a way that causes distress to their spouse, I think that can be sin. It's hard to say what this is, but if one partner requested something from the other that made them feel degraded, or if they have to be pressured into it not really willing, then that can be sinful, I think. I'm not talking about one spouse requesting something the other spouse may be unfamiliar with and need to "get used to" it but isn't really opposed, but more like almost forcing.
I do think porn is a sin, for the reasons mentioned in the above post.
As far as various physical acts, explorations, play-acting, cameras, costumes, toys, etc. ... I don't see anything inherently sinful in this, as long as both consent willingly and no one is hurt. (although doing anything mentally that would be a sin - deliberately fantasizing about adultery for example - would probably cross the line into sin)
It should go without saying that sex should be between the couple, and no other person should be involved of course.