wow, there have been some excellent answers here so far and i think it just goes to show that in this, as in all things, we will all differ...
Is your marriage a happy one?
most of the time it is, yes but not always. i don't think that any marriage will be 100% happy, 100% of the time.
Why do you do it?
i don't have an online Master / significant other but i do participate in a fun RP with Oz and have recently joined the Academy and will be taking part in tasks there so i suppose that counts. my Master waivers in this lifestyle while i do not. these activities keep it real for me in His down times. it is difficult for me when He turns himself on and off as my Master, though i continue to treat Him the same all the time. these activities help keep me grounded in my true self. that's why i do them.
Would you leave your spouse for your D/s partner? Would you meet them in r/l?
to be honest, i don't know. it would depend on the other person and the situation as a whole. i love my Master with all my heart - we have been together for 14 years, married for 10 and most of the time, our relationship fulfills me...the key here is the words most of the time. the other part of the time is an ugly time of readjustment, failed expectations, etc. if i was to find someone who i felt could keep me fulfilled all the time, and thus i would live a happier life overall, i would probably consider it. i realize that sounds harsh but i wanted to be honest. of course, i like to think that my Master and i are soulmates, meant to be together forever with nothing that can come between us but the simple fact of the matter is that i don't think anyone can answer that question until they are in that situation.
Is is cheating?
i don't consider my current activities cheating, no. my Master is aware of what i am doing though not necessarily every word that is spoken, every feeling that lives within me during that time. i have not given my heart to another. yes, i have allowed another to instruct me to perform an act that involved sexual currents but my Master was aware of the task, had read it and was right there when i performed it.
When does it become cheating?
for me, it becomes cheating when either one of is hiding something...obviously it is being hidden because we have strong feelings that it would not be appreciated by the other party. there is so much more to this than meets the eye - these are difficult questions to answer. i dont think they can be answered simply.
just my 2 cents
respectfully,
orchid