Okay...
1. Puzo CLEARLY indicates that Mike was hit by the thunderbolt... love at first sight... what have you... they were in lust or maybe even love... as most people would agree, love over comes most of your other emotions.
2. You people have to realize that Mike isn't exactly a saint, he is a murderer... he shouldn't be expected to make rational decisions when it comes to women... let alone other people.
3. Kay KNEW what she was getting into, if the new lifestyle was so different and tough for her to adapt to, then she should have found somebody else.
4. I hate to say this because I like to get into movies as much as most of the people on here... but really... does this all bother you THAT much? They are after all, FICTIONAL characters. If you are losing sleep over what a few people on the internet believe then I'd recommend you get some help.
-Lucky