People often ask me, because they are married and they have decided to stop the affair, if they can be friends with their lost loves. With very few exceptions, I have found that the answer is no.
If the two have had a sexual affair, and they are "separating" only because they are married, then they don't really want a friendship. It would be a romance without the sex. Sure, this works for some simply flirty "romances" (the kind you see on TV, when the two detectives have an unacknowledged attraction to each other... but that's TV), and there are plenty of good marriages that are no longer sexual, but if the lost loves are just cold turkey on the sex, then one or both of them will want more.
The more contact they have as "just friends," the more likely it is that one will get resentful of the other for withholding sex from the mix; I have seen reunions turn very ugly because of this kind of resentment, frustration.
If the two lost loves were romantic, sexual partners when they were in their initial romance years ago, then they were not "friends" then and they do not suddenly become friends after they stop having sex later in life.
Certainly a good romance is a good friendship, too, but that's not what I am talking about. When people ask me if they can be friends after the affair, they mean can they be platonic friends like any of their other high school/college friends are. No, they can't. The sexual attraction is too strong and they really crave more than a simple friendship. They just don't want to let go and so they think calling it a friendship and withholding sexual contact can preserve their connection to each other. It can't. It just makes things worse for one or both of them. And when spouses know and have asked that their husbands or wives have no further contact with the affair partner, then the friendship plan becomes even more untenable.