Part-time lover, full-time friend

Last week, I went to see the movie "Juno" with a friend. It was cute, although I don't quite understand the huge hype they made about that movie, it's not that great... but what I want to write about is not the movie itself, but one line in a song they perform in it.
I guess you know the feeling when you hear a song or read a sentence in a book and it just touches something deep inside and almost makes you cry - and I had such a moment in this movie. It's about this teenage girl who gets pregnant (but decides to find the perfect parents for adoption for the baby), but needs the entire time of her pregnancy to actually fall in love with the father of the baby, who is one of her school mates. In the end, obviously, they get together, and they sing this song which starts with "you're my part-time lover and my full-time friend" - and before I realized what was going on, I felt a tear running down my cheek. This one line expresses so perfectly well what I miss so much, what I hope to find again one day, and what I know is something extremely difficult and fragile, but worth fighting for. I have so many "full-time friends" (also male ones) around me and even have kind of a "part-time lover" (without wanting to go into details here, it's probably one big mistake... :)), but I know that, in order to be really happy, I'd have to find a person who'd play both these roles at once. It's just that I am afraid of what is at stake: I know that when I get to know a guy in a club or somewhere, the chance that he will become my "full-time friend" is rather small. At the same time, letting something happen with a close friend - and thereby turning him into a "part-time lover" - would most likely destroy the friendship, unless he's also serious about it... But how can I know when I've found the one who would fit into both roles?