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.

Also not a prostitute, also heard them answer this before.

They often pick something that makes it clearly different. Like always use condom at work, never outside work. Maybe only the boyfriend gets anal. Or maybe only the boyfriend gets vagina.

Actually the condom thing was an issue mentioned in the news in my country. That is not a good strategy, especially if they get a new partner fairly often.