Preface: I am married, happily so.
Story No. 1: Recently, whilst walking down a hallway at work, my thoughts on the possibility that I wouldn't have to wait for someone else to evacuate the washroom, I happened to pass a very attractive man. He works in another part of the building so I barely knew he existed. Anyway. So I'm walking, I see him, and bam! it's a crush.* Just walking, minding my business, and then crush.
Story No. 2 (and even more pathetic): H showed me something on his friend N's blog that was funny. So funny, in fact, that I continued to read N's blog long afterward. Bam! Crush on a guy I don't know, whom I've neither met nor seen, and who is (for all intents and purposes) a friend of my husband's.
Aside: I don't go looking for these things. I don't wake up saying, "Hmm, I'm unsatisfied with my husband. Maybe so-and-so can fill this void." Nope. Usually all that happens is a funny feeling in my abdomen and a sinking sensation in my brain, an increased pulse rate, and the thoughts of "Oh, crap, not again."
Supposition: Crushes must have some kind of evolutionary basis; e.g., when a certain number of (pardon my cranial phonetic ignorance) brain cells are misaligned—because obviously there's no logic in crushes—they create this psycosocial construct, an evolutionary construct designed so that the species prospers and multiplies, &c.
Ah, but the last laugh will be mine: I have no desire to reproduce. Mate? Perhaps, but not produce little humanoids. Take that, nature!
*Bonus points to anyone who can come up with a better, less juvenile word for "crush."