The 3rd Precept and 'mia nois'

I have a question about the 5 precepts, especially the third. As this situation can be seen often here in Thailand it would be interesting to understand which of the below cases actually breaks the third precept (my interest is theoretical, I am not in this actual situation myself).

A man has a girlfriend or wife and cheats secretly on her with another woman; lets just assume his mia noi is actually satisfied and happy in her role. Still, clearly he breaks the 4. precept (false speech and lying). But does he break the third precept?
Now lets assume he does not cheat on his wife but tells her the truth; he might assume that his wife is emotionally hurt, even though she does not say so. Now he is not even breaking the 4. precept.

I am aware that the Buddha warns against this behavior as immoral in the Sutta Nitaka ("Not to be contented with one's own wife, and to be seen with harlots and the wives of others — this is a cause of one's downfall."), but none of the precepts seems to be broken.

Discussion here on the Thai Visa Buddhist Forum and on e-Sangha.