I know you're keeping the thread focused on one particular question, but it got me thinking about one I've been mulling over for a while and it seemed to fit.
Why do some mono guys see no threat in their female partner dating another woman as well?
One conclusion I've come to is that men feel no sexual threat from women... like there is no competition, which sort of makes me smirk. But that's being filtered through my lady-like b.s. filter, I'd like to hear from men that really aren't put-off by their girlfriend dating another woman!