I know several happily married couples with kids and everything, that started as friends with benefits or even just sex. As in basically sex before the first date. So yeah, totally possible.

For your second question, the easy answer is that it’s how they were taught they are supposed to be. On purpose or not. Maybe both parents didn’t like talking about emotions with “a boy” or with anyone, and now they are not used to it and barely have the language to talk about it. Or maybe something made them believe that their emotions are unusual, weird or even wrong, and choose not to share. I guess it’s very individual.