Your views are on the extreme side of most, and you are entitled to those of course. I personally disagree with you.

Marriage to me is not something you should stay in just because you've vowed in front of people and god. I'm not religious, but have had my fair share of it over time and think that if either partner feel that they can no longer stay in the marriage they have every right to leave, not to just put up and shut up.

There is indeed to much marriage and people today don't always think of it seriously enough but that is their choice, as is divorce and that is what free societies are out.
I don't see divorce as the evil you do, people change over time, as do the relationships they have with people during their lives. In fact, when my parents divorced it was (in retrospect) a good thing for my mother and our family, my parents' relationship was bad and now in the 13th year of her second marriage is happy, as are her children.

Seeing all the vanilla people getting divorced "just" because he cheated and finding it silly, ever since I was 9, my firm view on that was:...as long as he doesn't fall in love with whats above the neck, sometimes sex is just sex, isn't it? What would be the deal breaker...is him lying, or even not telling me about it. Honesty and loyalty, I value above all else.
I completely disagree, vanilla or d/s, cheating (as defined by the couples relationship) is cheating. As you say, honesty and loyalty you value above all else so why is it that as long as there is no love toward the 'bit on the side' it's okay. In my opinion, the cheating party has been dishonest and has not been loyal!

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, and it applies t women also.
How do you deal with infidelity (in more conventional relationships),...especially when there is an added element of bdsm involved?
The same way you would deal with it without the bdsm, I don't see a bdsm relationship any differently to a vanilla one. I may be the submissive in my relationship and my role defined by me and my dominant but our core relationship, the love, the trust and what keeps us loving one another is no different to my parents, who are not d/s
What do you consider infidelity in bdsm? Is there such a thing?
Those things are defined in each relationship, in mine it is boradly: We can both play (floggers, canes etc etc) with other people but sexual contact is forbidden. This is something we both agreed on.
Or does the dominant get to do what they want? What about sub?
No, he can not do anything which I've said no to, as I can't do anything he has said no to. Just because he is dominant does not mean he can veto my feelings.

You seem like your religious morals and society are clashing, as is often the case. Let the world do what it wants and don't think about it.
I've given up trying to understand society, as long as I don't disappoint myself, or my partner and we are happy in our lives I am happy and content.