I think it is highly unlikely I will ever get married, not because I am a "loose liberal infidel" (as I let most believe) who doesn't believe in it, but because I believe in it too much.

I have never been a fan of marriage, only because I believe it wrecks a relationship. I'm not saying that it does for every marriage, but the ones I have seen go sour all too quickly. I just don't see the point in it.

I don't understand how you can stand in front of God, or people, give your word, make a commitment...then one day take it back, just because its no longer convenient, because you have found something better, because you have to go and find yourself? (And I question that regardless of what religion and deity you subscribe to, even atheists believe in something.)

I don't think that people just one day decide that things are no longer convenient. I believe that most who do wander away from commitments do take the time to think carefully about thier situations. Maybe some don't, but I would believe it's a very small minority.
People change over time, so do thier ideals, thier convictions, and thier lives. Some decide to move in a different direction, others opt to stay and work through changes. It's all in what a person wants for themselves.

Which brings us to the matter of infidelity...

is it really too much to ask of men to be sexually, as well as emotionally, faithful?

I don't think it's always the men who are the culprits, I don't care about the sexual aspects, I only care about the emotional and ask that my partner be honest and forthcoming with me.

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

Again, I don't think you marry someone with the full intentions of being dishonest and untrustworthy, as I said things can change.
If you go into a marriage with full intentions of destroying it, I would have to ask the same question.

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

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

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

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

What is considered infidelity in polyamorous relationships?

Infidelity is infidelity, how you handle that is your own business. I don't think that infidelity in BDSM relationships are any different in vanilla ones.
There is an obvious breakdown of trust and communication if it is prevalent.

If a Dominant or submissive wants to play with others, or be involved in multiple relationships than it should be discussed with all parties involved. It's not infidelity when everyone is on the same page.