My friends from high school are in that zone. If we'd hooked up in school, it might have been repeatable when we were adults and single. However, we didn't, and by the time we reconnected three and four years after high school, it just wasn't going to happen (even though they all came back from college HOT). Every moment spent together solidifying our relationships was a moment further away from ever having sex. In high school, I just wasn't interested. All but one of them had expressed an interest at some point. However, they were all still in their awkward stages. When we reconnected a few years later, if they'd still been interested, I might have responded positively. Then again, I knew all their embarrassing details, and stupid nicknames, and had gone through a lot with them. With no romantic love in my heart for them, it might have just been awkwardly like fucking a sibling. That is what the friend zone is for! To protect myself from that type of conflict.
Separately, there was a group of friends I made specifically for fucking. Over time, they grew to be real friends. Some of them know my husband. Some of them check on my grandma for me now that I live 1200 miles away from home. I will never sleep with them again, and though they have missed that aspect during the past few years, they remain good confidantes and emergency dates (since my husband is often away at work when I need someone to go to a wedding or party with me).