He wants, she doesn’t
The assumption: that in any discussion about marriage and babies, she’s the one who wants it and would have his immediately if only he’d ask. But he’s the one who wants, or at least is leaning more towards having kids, than she is. HE is the one who wants to start a family with HER. She’s resistant because of concerns about her past health, worried about what that would mean for both her, and her future children. He’s not pressing but they’ve been seeing specialists together, looking into different options, hoping to perhaps be parents within two years, even though no one seems to (want to) believe they can make it that long.