There's also things like:

- Constantly being accused of an affair, so choosing to have the benefits of one too.

- Revenge (often from the partner having an affair, but not always)

- Addiction, nymphomaniac

- Lapse of judement from alcohol consumption

- The thrill of doing something secret (while assuming your partner would never leave you)

- Pheromones that drive one insane. A different type of "lapse of judgement"

- Pity sex

- Curiosity

- Boredom (this is often related to lack of attention, but not always)

- Fear of missing an opportunity, usually with an old flame that never happened, but not always

I guess you got me to list most of the examples I've known about.