The Trouble with People... not in love.

There's a general philosophy among many people, whether consciously or otherwise, that over time feelings change and that sooner or later this person with a crush will inevitably find someone else who will return their feelings and will forget about the way they felt in the first place.

It's a very cavalier, "We'll look back on this and laugh," attitude. Only, after the Earth makes a couple trips around the sun, people find they never laugh when they look back. Matters of the heart are not silly clothes worn in the heat of a trend. They stick with you long after you've replaced all the things you wore and even owned at that time.

Some people have yet to understand the expression, "I could never see the forest for all the trees." You can't see what else is around you if you're only looking at the objects placed in front of you. It is difficult to recognize when you are the object; that takes maturity and depth.

I will assign gender roles here, but please note that the same message is equally true in the reversal of roles:
If a girl remains friends with a lot of men who have made it clear they want to get in her pants, maybe she really likes the attention - even feels validated by it, somehow. Who wouldn't want to be surrounded by people who flat out adore you, just love to love you, and cannot get enough of you? Or, perhaps these guys aren't in a bad place to remain friends because their attraction is mostly physical and their friendship would have taken an incredibly awkward turn after they boned, anyway. Deep down, they know it, and would rather hang out with an otherwise cool chick who may have single cool chick friends they can bone without care.
Maybe she gets off on having "back-up admirers;" men whose laps she can "innocently" sit upon whilst her honey is somewhere else, doing God-knows-what.
I've seen it among couples of whom aren't that serious. If someone is engaged and behaving in this manner, then my guess is that he or she has become accustomed to attracting only physical attraction from others and lumps every admirer into that category without much consideration to what their intentions or hopes could be.

The whole scenario is a bad situation simmering on the back burner of that person's mind.

Be no one's back up.

Find someone else to spend time with, and that's my advice - whether or not you asked for it.