Is it a red flag if a man is no longer in contact with his children?

i am with a guy who is divorced and has two kids from a prior marriage, which he had never told me about. I discovered it by myself on my own. we are currently 6 months into knowing each other, moved in together. Recently he asked me to Marry him. He has narcissistic personality disorder.

When I questioned him, He said that he divorced his ex wife because he wanted to have sex 2-3 times a day while his she only wanted to have sex once a day. But I find it kinda strange how he no longer has any contact with his kids. It's like his ex just completely disappeared with the kids. He didn't even bother to fight for visitation. I have a feeling there is way more to the story than what he is telling me. I also never told him that I know he has 2 kids. He thinks I am unaware of it , as long as he keeps his mouth shut.

As big his flaws are, I can't live without him. But at the same time, I don't want to make a mistake into marrying him. I would love to dig up his ex wife's phone number and have a talk with her about what really happened. I don't want to Marry someone that I would end up regretting just like her

Is it a red flag if a man is no longer in contact with his children?
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