The idea is that they know it's business, not personal. There's no potential hidden agenda about trying to bang your co-worker that there might be if you were single.
Because adultery is unheard of? I am not denying that when you are married or in a known committed relationship the tendency is to interpret things in a purely "two co-workers going to lunch" kind of way, but to assume marriage guarantees that end goal, at least in America, is probably a bit hasty.
In particular: "so many of the attitudes her work unveils are of an 'unconscious nature,' which makes beating them back particularly difficult" and "male leaders may think they are elevating women, not stifling them."
There's other research suggesting that "being made aware" of the problem helps male leaders adjust behavior.
There's significant pressure to not talk about sexism directly and just address this as a conflict between two individiuals (see antirez's rant) -- even though the research says that talking about about sexism and fairness concepts changes the behavior of management.
You may be the kind of guy who can do that, but after a couple of those outings one of you will probably start to develop affection for the other one, and that is why I would recommend sticking to a group.
I would think that it would make it MORE awkward for you to ask a female colleague to go out.