I can understand the wanting to have a "guys night", regardless of the theme. And I agree, as themes go, D&D night is probably one of the least threatening and most acceptable from the standpoint of a significant other, as long as they like your geek side (in my experience most do, even if they use it to tease you

). I'd think from the standpoint of most significant others, gaming definitely beats barhopping and other less respectable "guys night" pastimes.
If you and the guys in your group prefer a "guys only" game night, I don't see a problem with it. If that works for you guys, then play on.

Now, thinking that a "guys only" format is the absolute best, for every gaming group, I'd have to take issue with that. I'm not saying that's necessarily what you intended to say, but that's how your OP came across (at least that's the way I read it, and it seems like some others did also based on some of the above posts).
I've played in both types of games. Being in the military, and being deployed or stationed overseas while single, I've played a few games that happened to be only guys. However, it wasn't an intentional choice, just the reality of the location (and I'm not saying there aren't female members of the military that aren't gamers also). The first game I ever played in had two married couples there every weekend, a third that occasionally played, and four or five single guys that rotated through the game (actually everyone kind of rotated through the game, based on who was deployed at the time and who wasn't). The last game group I was involved in had three couples as permanent members and only one guy who gamed without his wife (she just didn't get gaming, but it was cool with her for him to play). Even though the "all guys" game was a really good game (more because of the game itself than the group), the groups I've had the most fun in have been mixed groups with wives and girlfriends.
I'm not trying to say that your ideas are wrong at all. Just don't close yourself off to the possibility of such a game. In my book, nothing makes the game better than a variety of different approaches and viewpoints within a game group (at least to a certain extent, there are certain gamer types that just don't mix well with eachother, but that's a different thread entirely

). Now playing in a group with a significant other that really doesn't like gaming, but is only doing it for some other reason, usually doesn't work very well. In that case I prefer she doesn't play. But, I find that's an exception rather than the rule.
It's no great revelation but, men and women think differently. That diversity of thought only seems to add to the fun, I've never seen it be detrimental. Besides, they smell nice.
