[brokenmachine]

> It's considerably less hurtful to just leave someone than to carry on a hidden affair.

Not if nobody finds out about the hidden affair. But yes, it's a douchey thing to do.

> I guess the optimist in me believes that if a partner is no longer in love and no longer interested in trying, they should break up rather than cheat.

That doesn't sound terribly optimistic to me, optimistic IMO would be believing that a vow for love forever might have a chance of working.