From what I've read on this (provocative) thread, it seems to me that most of the contributers are from the western/catholic world. I don't want to hijack this thread but with all due respect, why not MARRY the second woman? I know, I know... most if not all women on this thread will frown and say... phahhh! But if you're not strictly religious (catholic) and possess a free mind, then you can look into other cultures for a solution to this chronic problem that affects a great portion of men in the western world.
In middle eastern culture, a common practice is for the man to take a second wife and share his time/wealth/love with both of them. Sounds absurd? I have several friends who have 2 wives who have accepted each other and are on good terms.
Without getting into any religious debates here, IMO it sounds to me like a solution to a problem that has no answer in western culture, both parties willing of course.