Should I reconsider marrying him?

My boyfriend proposed to me yesterday. What was a beautiful day, quickly was ruined after my phone conversation with my fiancé on his way home from work. I told him my best friend told me not to post the announcement yet because she was sending me a gift that she wants me to post with it. He then stated now is not a good time to post anything, and began speaking about his ex wife, and how he didn’t want her to see. I don’t even have her on social media and my page is private, so I’m not sure why he felt the need to bring her up. He started to say he was scared she may try to take their kids away or have some devious intentions. They divorced 5 years ago because she got strung out on drugs and cheated on him. My fiancé is active duty military so their two sons live with his parents full time. They only see the mother on the weekends. I’ve met his children twice and they love me. They even told my fiancé they like me much better than their mothers boyfriend the first time they met me. I never met her, but I don’t like her from what my fiancé has told me about her. She constantly begs him for money, that has nothing to do with their children. It pisses me off and is the only thing my fiancé and I argue about. I’ve told him multiple times she’s taking advantage of him. He complains because if he doesn’t send her money, she threatens to take their children away. I don’t believe it, because if she wanted full custody of their children she would have been fought for it. He recently sent her $300 for her car payment but expressed she needed to pay it back. She claimed she would, but never did. It’s now been three weeks and he hasn’t heard from her. Even though he responds IMMEDIATELY when she reaches out to him. I know she’s a trifling woman but he constant defends her. He claimed she was such this good woman and mother. Well if so, why would he be concerned of her having devious intentions because we are getting married? Could there be something he is hiding from me?

Should I reconsider marrying him?
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