I cheated on Kaiden because I accidentally fucked Garrus.
In all honesty I accidentally opened up the love quest line and didn't have the heart to close it, because it's Garrus and Garrus is awesome. Also, being bitched out by Kaiden for something that was out of my control (AKA being dead) and then having him exclaim something along the lines of "WHY DIDN'T YOU CALL ME?" while I was dead, was not fun. He also did not accept this explanation, even though he was on a special mission that the Alliance and Captain Anderson specifically kept me from talking to him or even mentioning to him that I was alive. He should have known this. So yeah, retard. (Also I should have gotten the dialogue option to mention to that to him while I was there.)
Also, I worked with Cerberus because they were trying to save the galaxy and I didn't expect Kaiden to question my good judgement. (After all, I didn't leave him to die the first time through, though I did the second time and will every time after this because he was being a whiny little jackass.)
I will however remain loyal to Liara because she behaved like an adult. Kaiden and Ashley do not.