Unless you're trying to annoy your readers, you probably shouldn't spend the first third of a novel building up the UST between two characters, only to have the female main character MARRY SOMEONE ELSE HALFWAY THROUGH especially when YOUR READER HAS NO REASON TO GIVE A DAMN ABOUT SAID OTHER PERSON.
I mean, think about it. If midway through Pride & Prejudice, Lizzie forgot all about Darcy and married some guy named Bob, would we will be talking about it all these years later? I THINK NOT.
Oh wait, I have bonus advice!
Writing 2/3 of the book in third person and 1/3 in first? Lame, man, really lame.