Disagreements between spouses?

We don't have an adoption situation, but I'm hoping someone here will be able to give me some advice. I'm biomom to 4yo DD. She's being raised by me and my partner (DP) of 14 years, who due to state law has zero legal relationship to DD. When DD was 2, we were contacted by her biodad (formerly an anonymous sperm donor) from a listing I had placed on a sibling registry. DP and I never really discussed bdad or how to handle explaining him to DD. (Yeah, in retrospect, that was a poor decision. We were also having relationship problems, since mostly resolved, and have concluded that those problems led to many, many poor decisions. The aftereffects of which we're still working on.) So when she had questions, I answered them, and I included the fact of bdad's existence in them. So this week, DD starts saying to me that she wants to visit bdad. And she dictates to me a letter for him that says she loves him and invites herself to his house. At which point DP says (privately, to me) that if she had her way, DD would have no information and no contact until she was 18, at which point she could do what she liked - which DP assumes will be to abandon DP and run off to live with bdad. My position is that making bdad normal and boring, rather than letting her build up fantasies about him, is the best way to reduce the chances of her abandoning anyone to run off to live with him. I'm guessing probably no one has BTDT for all of it, but maybe for some? Any thoughts or suggestions?