In romantic comedies there is usually the other. The Bellamy, the Baxter (as Michael Showalter has labeled him), the Pullman. The character who's sole purpose is to show that the female (or in some cases the male) is attractive but not available immediately, allowing a narrative to stretch from meet cute to relationship. This is evident from the credit sequence, as this character is usually fourth or fifth billed.
Many writers take to marginalizing the character. Nora Ephron's solution was to give the guy three scenes, and then have him gracefully bow out as his finance runs off for Tom Hanks. Much as in how Team America still seems discussion worthy even though it's not a great film, this sequence achieves a certain poetry. One wishes they spun off this character, so we could see who this guy is. Perhaps that's just the definition of bad writing, but Ehpron created a character so interesting in spite of herself, that I can see why Showalter was interested in exploring this archetype.
The other way to go is the direction of Wedding Crashers, a film that makes the secondary love interest so repellent, that we can't help but root for our hero to fuck up their relationship. Of course the film then asks a bigger question: if our heroine is with this lout, then maybe she's not as awesome as the film makes her out to be. "She's perfect, except she's dating a total retardo... But she likes me." Lubitsch always managed to work around these problems. Then again, he was a genius, something I don't think could be leveled at Ephron or the men behind The Wedding Crashers (in Showalter's case, a wait and see method is probably best), which in the later case is a perfectly fun though disposable romantic comedy (I think it'll end up more like Old School than Caddyshack).
But the problem with the character is that by its nature it must be a straw dog. To give them any depth would be to open the door to hurt feelings. It's a fascinating conundrum.