She was dating a married woman
Pertaining to your question, I would say that he owes you little or nothing, which is actually good because I suspect that any explanation he could give you would be fractional, confusing, and possibly self-conflicting.
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In fact, if you just give many married women a little nudge, you’ll find that it’s incredibly easy to date the married woman you seek.
It’s the fact that many married women know how to please themselves and the man that they are with.
For lots of guys, this is just what they want and need.
This is almost always a mistake: first, you should bear in mind that she has most likely started an affair with you because she wants excitement and danger, and not to start another long-term relationship. So, by all means enjoy yourself, but make sure you don't become too attached - above all, it's bad affair dating etiquette!
Also, remember that if you are a cheater, and you are dating a fellow cheater, this may not be the perfect formula for a committed, loving relationship.