why did Sean have an affair with Monica?

It seemed odd seeing how everything was more or less fine with his relationship this time around. He was under a lot of pressure with his kid on the way but that alone didn't feel enough for him to cheat on his wife yet again. Was that the point, that he learned nothing? that they both did? that the baby wouldn't fix the problems in their relationship and history would repeat itself? that's the only thing that makes sense but the execution still feels forced.