But recently, while still holding that belief, I began to question - is it really too much to ask of men to be sexually, as well as emotionally, faithful?

No it's not to much to ask, if it's what they agreed to before the relationship began.

Why would you marry someone with whom you are not honest, someone to whom you cant trust?

Maybe the person didn't realize they were being dishonest until after they were married, and maybe they were just scared of the truth and didn't want to see it themselves.

How do you deal with infidelity (in more conventional relationships),...especially when there is an added element of bdsm involved?

I don't, there's no such thing as infidelity in my relationships as I don't demand my partner live by my rules.

What do you consider infidelity in bdsm? Is there such a thing?

Depends on the relationship, if the relationship is structured so that there is one dom and one sub and it's agreed upon that there will be no additions, then adding someone would be infidelity.

Or does the dominant get to do what they want? What about sub?

Again it depends on the relationship, however a dom does not get to cheat simply because he's a dom.

What about domming and subbing others, is that considered infidelity?

That depends on the structure of the relationship, I've been in relationships where it was, and others where it wasn't.

What is considered infidelity in polyamorous relationships?

I don't do poly relationships, so I'm not sure.

When it comes right down to it, every relationship is different because every person is different, what works for me isn't going to work for anyone else, because their brain is not a carbon copy of mine.

I'm a pretty open person, I don't expect my partner to be with only me, and I don't care one way or the other about it. When it comes right down to it, it's a security issue for me. I'm secure being me in our relationship. If my owner wants to go screw/beat/order around/ or tie up another piece of meat for the night because it's amusing, that's cool. My only stipulation is that it's done safely and that I know about it, so I'm not blind sided by "by the way your owner is a great lay" later and have to look stupid.