Why would a guy stay in a relationship with someone he doesn’t want to be with?

I dated this guy for 2 years we lived together we talked about having a family and spending the rest of our lives together he would always mention proposing to me. I thought I found my soul mate and he’s the only guy I could ever fully open up to. Later into our relationship I discovered he was still involved with his child’s mother and had feelings for her. I left him but he came back around with a lot of excuses saying she was bitter and he couldn’t see his son so he went back to her temporarily. I took him back after 6 months of him pleading and I got pregnant the next month. Before I told him he left me for his ex again and told me he wasn’t leaving me for her he just know I can’t trust him and he has to communicate with her to get for his child.(I never had a problem with him communicating with her I just wanted access to his messages to make sure nothing else was going on) and when finally told him I was pregnant he smeared my name to his family told his ex my personal business and pressured me into having a abortion. When I had the abortion I cut him off completely. it has been a year and we reconnected and now we are friends we communicate on social media and I only seen him 2-3 times.

I asked him a few times why would he waste my time and be with me if he still had feelings for his ex he said he didn’t and he don’t know why he did what he did not really answering my question. why would a guy be with someone else if he’s not over his ex I really thought he loved me and cared about me but he didn’t
Why would a guy stay in a relationship with someone he doesn’t want to be with?
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