Love At First Sight/Fan Works

DC Nation had Beast Boy and Fauna meeting each other in the middle of a battlefield, stark naked, and in full-on shape-shift mode. While not the most romantic of meetings, it certainly turned out well for them.
Luminosity has this as one of the things that simply happen to vampires and werewolves. This can be interesting if the other party is human, as they aren't affected, but generally they end up being turned—automatically mutual—or getting attached to the sheer devotion, respectively. Half-vampires whom vampires fall for get neither perk.
Many, many Captain America/Iron Man slash fics have Tony falling in love with Steve the moment the Avengers free Cap from deep freeze.
In My Inner Life Jenna sees Link for the first time and "And then I fell in love."
On A Cross And Arrow Fluttershy and Rarity both fall in love with their male alternate-universe counterparts (Butterscotch and Elusive, respectively) instantly.
- In Rarity's case, she actually invokes this trope. Claiming she knows herself well enough to sedu... er... deduce that Elusive would be "the perfect gentlecolt" to her "fair lady." Turns out she was right as she knew what subtle cues and innuendos to give. Elusive actually picks up on this and takes their tennis match flirtation Up to Eleven when he finally asks Prince Solaris to what could amount to a marriage proposal!