No one can be "stolen" without their own consent. Plus, he had feelings for her before.
You can choose not to accept the situation but it won't change anything. All your refusal will do is make you miserable, it won't bring him back to you.
You're obviously educated and intelligent. Many men would want to date someone like you. But you'll never meet any of them if you insist on focusing on someone who doesn't want to be in a relationship with you and who is actually with someone else.