My favorite was the second couple, as it was the least cheating-wise and I've always had a weakness for tsundere's.
I'm pretty sure the guy there was the son, as the husband was noted as living separately. His reaction to seeing his mom and best friend make out was amusing.
Thankfully this is just a preview book. Waiting for the rest continuation of each pair...
Btw, am i the only one who liking the first pair? They're interesting to say the least, though...