Christ on a cracker, people, do you know what CHARACTERIZATION is?!
I know many of you do not like McGee. He's a walking stereotype of a nerd. And, somehow, he's managing a relationship with Abby, who's... well, intelligent, but definitely not a nerd. She also seems quite happy with the relationship.
So for the love of all that is good and decent in fanfiction (not that there's much anymore), why would you ever think McGee would cheat on Abby? He's obviously head-over-heels for the woman! He'd be more likely to scare her off by proposing to her!
And if Abby ever did catch him with another woman, she would be more likely to hand him his balls on a platter! Then, when Gibbs, Kate, Tony, and Ducky found out, he'd wish Abby had killed him!
In other words, unless McGee has been replaced by a pod person, find a more reasonable explanation for your "OMG McGee is EBIL and only the TWU WUB of Gibbs can heal Abby's HEART!" fic.