I think that with everything going on in this scene we missed the fact that Bruce and Natasha were so far in their "relationship" that Bruce was already thinking HAVING CHILDREN with Nat. He literally thought about having a future with her, even when they weren't something "official" but he's sad about the fact that he can't offer her the future he desired for them both and he KNEW she also wanted. How did he even know Nat wanted something long-lasting if they didn't know each other that deeply. I feel like in the movie you don't see it that clear but if you really analyze everything, they were deeply in love with each other. Nat thought about having a whole life with him too, even if that's not what she ever pictured for herself with no one else, that's why she told him in the extended scene that they shouldn't play circle of life and just play.
