Why is it wrong to date someone who has left their partner for you?

As my other posts stated, I have 4 kids and was dating a man that turned out to be married. I been with my other 2 kids father for more than 11 years and my other 2 kids father was killed when they were young. I met this man who I had a feeling was cheating on me. He wouldn’t tell me who she was or anything about her. He said he left me for her. He told me yesterday to tell my kids to erase him from their memory. He wrote my daughter on Facebook and told her to remove anything of him from their Facebook page. I reached out to him through my sister. My sister texted him asking him about not leaving the past in the past. He responded that his wife wouldn’t take him back at the time and they never divorced. He told my sister to stop texting him and to leave him alone. His wife is currently pregnant with her first child. They were together for almost 11 years. He wouldn’t let me on his Facebook page or post pictures of us. He told me that he couldn’t add me on Facebook because of his job

Why is it wrong to date someone who has left their partner for you?
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