You say a lot about your daughter’s state of mind—that she wants marriage and children; that she’s having a good time in this relationship; that she “knows the relationship is going nowhere.” It’s not clear to me, though, whether she’s shared these thoughts directly with you or—like the information about her boyfriend not discussing his divorce with her—they’re coming to you secondhand (or are simply your assumptions).
Right now, your proposed strategy for communicating your concern and love for your daughter is through punitive action (boycotting her boyfriend). Sometimes when parents feel powerless, they resort to what’s essentially a hostage-taking situation. Until you do as I wish, I will withhold something important to you. But these tactics rarely work, nor are they “good for” your daughter.
You may not like this situation, but you love your daughter, and punishing her isn’t a way to show your love. Instead, it shows a need to exert control, to erase her personhood from the equation. You can’t love someone by erasing her personhood. And the more you erase her by insisting that she see her relationship the way you do, the less receptive she’ll be—not just to your thoughts, but also to you more generally. If you’re worried about your daughter losing a particular future because of this relationship, consider that you may lose a future with your daughter because of the way you handle this situation.
So let’s consider another way of addressing this issue between you and your daughter—because that’s really what your letter is about. You say that you can’t understand why she’s with this guy, but have you tried—in a sincere way—to understand? There’s a difference between an anxious “What are you doing with this guy?,” which will put her in the position of defending herself, and a genuine conversation that comes from an open-minded place of wanting to learn more about her inner world.
What she tells you may be hard to hear. Perhaps in an ideal world, she would love to have children, but she may feel that that is not a likely path for her right now. Even if she were to break up with her boyfriend tomorrow, she’d have to meet someone new very quickly, a prospect that’s full of uncertainty. She might not connect strongly with anyone for a long while (the dating pool is more limited at midlife, given how many people are married by then), or she could go through a series of short relationships that don’t work out—all while her fertility timeline shortens. If she eventually meets and falls in love with a younger woman, that may buy her time—and, of course, she can try to adopt children if she ends up with a same-age or older partner. But if she wants to parent with a partner whom she has yet to meet and then get to know well enough to spend her life with, she might be doing the math in her head and coming to the conclusion that having an infant at, say, 50 years old doesn’t appeal to her—especially when she’s currently with a man she loves. Consider, too, that in many people’s minds (including, perhaps, your daughter’s) there are a lot of ways life can work out that fall between having “nothing” and being married with children.