Sure I've wondered that about guys I liked and had a casual thing with when I wasn't in a place for a serious relationship. But then I remember that it's easy to fantasize about what *could* have been, but actually being in a relationship and building a future with someone is a totally different thing. Because I was just casual with that person, I didn't *actually* know them that well on a deeper level. And that makes it very easy to romanticize them and think they're perfect, since you don't know all their flaws and their whole selves, which you wouldn't be able to know until you actually got into a relationship.