Better to be single than with someone who would throw away a serious relationship for some flings. At the very least be thankful she had the decency to end things with you first.
You’re subtly insinuating that the woman was being shallow or petty, when in fact, she could have just not wanted to move forward with the relationship.
Why do you believe that the woman owes the man anything after ending the relationship? A woman is not obligated to feel emotionally conservative just because her former partner does.
It's pretty likely that the relationship was dead for a while from her perspective. It's just that OP was blind to the signs.