Ah, yes. I personally like to call it "The Spare Syndrome."
Person X doesn't have a significant other but still wants some company of the opposite gender, and occasionally needs someone to call on to whine too or get help. Person Y comes along and fits just the right void for Person X... But Person X is not interested in them romantically at all.
Thus begins the saga of "The Spare." Person X will think, "I might not have someone right now (and am free as a goose to look around) but at least I can always call 'The Spare.'" The go-to fill-in substitute for the significant other you just don't have right now.
Of course, both men and women do this. The worst offense if when someone strings another person along into thinking it COULD be a romantic relationship, knowing full well they don't like the person that way but that the OTHER person DOES feel that way, but has no intentions of committing to a relationship with the person they're leading on.
My personal feeling is to get out of this situation as soon as possible, regardless of which role you might be playing, because it's obviously not sincere or genuine.
I actually wanted to write a thread a long time ago entitled, "How To Stop Being 'The Spare'", but just never got around to it.