Basic Trope: A character falls for someone belonging to a group whose members they'd never normally be attracted to.
Straight: Bob's in love with Charlie, even though he's never been interested in men before or since.
Exaggerated: Bob is extremely proud of his heterosexuality and will remind everyone of this fact every chance he gets, to the point of being extremely lecherous around women and disgustingly homophobic around gay men... except for Charlie, of course, with whom he's in a loving, committed relationship.
Downplayed: Bob just can't find any attractiveness in the members of his own gender, not even in himself... Except for Charlie, whom he considers the epitome of masculine beauty, and the kind of person he wishes he looked more like.
Parodied: Bob is straight, and Charlie is the one exception. Oh, and Dennis. And Edward. Frank. And George. And Harry. And Alan. Now that he thinks about it, he hasn't really dated any women. But he's definitely straight! We swear, honest!
Zig Zagged: Bob is known by everyone to be straight, until he starts dating Charlie. But then it's revealed that Charlie is a woman... but Bob didn't know. And now that he does know, he doesn't like her anymore. Finally, Bob comes out as bisexual.
Lampshaded: "You know, Bob, bisexuality is a real thing."
Invoked: Charlie is constantly teasing Bob with Gay Bravado interactions, all in the hopes that he might notice some day that he isn't just joking, but that he actually likes him.
Exploited: Alice thinks Charlie and Bob would make a sweet couple, so she sets them up on multiple surprise dates together in the hope that brute forcing it will spark something between them.
Defied: Bob only ever hangs out around women.
Discussed: "Isn't it weird how often straight characters end up crushing on someone of the same sex?"
Conversed: "So wait, Bob is bisexual?" "Nah, Charlie is just a special case for him."
Deconstructed: After realizing his feelings for Charlie, Bob has trouble redefining his sexuality to account for this single exception, and ends up deliberately sabotaging their relationship so as to prove that it would never have worked out anyway.
Bob recognizes the futility in categorizing something as complex as human sexuality, and decides instead to focus on the fact that he just really, really likes Charlie.
Alternatively, Bob realizes how flexible human sexuality is and that being heterosexual, homosexual, and/or bisexual is about which sex/gender you find attractive in general or overall and not-so-much one individual. So he decides to label himself as heterosexual for the convenience, but to always remember his sexuality is not limited to it.
Normally, I don't use the bottom links to return to the main page... But if it's to go back to If It's You, It's Okay, then I'll gladly make an exception.