Adoptive fathers relationship with birth mom

For the most part, most of my friends who are birth moms report having a strong relationship only with the adoptive mom of their birth child. Most of them don't seem very close with the adoptive fathers. In fact, some of them act like I'm weird because I have a really good friendship with my birth daughter's adoptive dad. He's like my big brother. Is this type of relationship out of the ordinary? What kind of relationship do you have with your birth child's adoptive dad, or your child's bio mom?