She obviously hates the subject, so he tries never to bring it up. It only takes one mention of, "In a few years, we-," before she's making an excuse to be somewhere else or remembering some urgent errant she has to run. She avoids his eyes and edges away from his hands when those words slip past his lips, and he knows better to chase after her when she's like that. He beats himself up for forgetting and resolves to try even harder to avoid the topic, but it's so hard when he's dreamed for so long about a future together, a family, growing old in each other's arms. Today she's been on edge about something and he's making sure to move carefully. He doesn't know what the problem could be, but he doesn't want to push her if she isn't ready to talk about it.
It lasts the whole day until they're lying in bed together and she picks up his hand to start playing with his fingers. He's wound as tight as a spring by this point from trying to guess what's wrong. She's only looking down at his hands, not meeting his eyes, and the tension's almost more than he can handle. Finally, with slow, carefully chosen words she says "Bo, I was thinking that if someday we got married-" He doesn't even hear the rest of the sentence. The blood is pounding too hard in his ears. He's sure he must be bright red, but there's a blush spreading across her cheeks too and she finally raises her eyes to meet his. When she sees the look on his face, she slowly smiles and he can't contain the happiness anymore. He pulls her into an embrace, and her arms wrap around him, holding him just as tightly. They're both babbling over each other and tripping over their words, but the one phrase he can make out from both of them is a repeated, "I love you, I love you."