Note to begin: If the person/people who inspired this thought process read this blog entry (unlikely, but possible), please don't take this personally - you just gave me some food for thought. Thank you.
Moving on. Everyone knows that, no matter how much you love your spouse (or Significant Other, as the case may be), there are times in life when you just need to spend time with people of your same gender, people who more or less
think like you do. Ladies needing "girl time," or guys needing "man time" (or whatever your group calls it) is understandable and perfectly okay. How this is accomplished, however, may need some review. The following options assume that there is a goal of a gender-specific get-together of some sort. We also assume that your Significant Other gets along with the S.O.s of the other people in your group. Ready? Here we go.
Option 1: Regularly scheduled meeting, probably on a more-or-less weekly basis. 
Examples could include gaming groups, singing groups, lunch dates, etc.
This is, in my opinion, the best option. Your significant other knows
exactly when you're going to be having your gender-specific time and can plan appropriately, perhaps arranging for some gender-specific time with his/her friends or pursuing a hobby you find annoying since you won't be around to be annoyed. It's a beautiful thing.
Option 2: Call up your buddies and say, "Hey, let's get together next week and do X." This isn't bad - gives some time for you to sit down with your S.O. and find something they would enjoy doing while you'll be away. You also have a specific plan so your S.O. knows you're not getting into trouble (assuming you've got the whole "relationship of trust" thing running smoothly). If you're too spontaneous/flaky for Option 1, this is the way to go.
Option 3: Call up your buddies and say, "Hey, let's ditch our S.O.s tomorrow night and go have some fun." Even setting aside the attitude issues,
this is entering the realm of tacky for several reasons. First, there's no plan for either of the concerned parties and probably won't be until you meet up. Once you get together, you'll probably spend half the time trying to figure out what you want to do, assuming you don't just give up,
order a pizza and tell stupid jokes/gossip until you pass out, several pounds heavier and not a ounce wiser. If that sounds like your idea of an ideal get-together, power to you and best of luck. Second, there's no time for the S.O.s to make plans. They'll probably hurriedly get together and be in the same less-fun-than-it-would-be-if-you-planned-it-out state as you are. If not, they'll spend the evening by themselves, probably doing something that they enjoy, but it's still tacky.
Option 4: Call up your buddies and say, "Hey, let's dump our S.O.s off to go do X while we hang out." This is bad. There's worse, but this is bad. You have the audacity to set the destiny/plans of not just
your S.O. but of
every other S.O. from your group of friends, doing
something which, if they really all enjoyed it,
they'd probably be getting together to do anyway, while leaving no plan for yourself. Basically, you'll be doing the less-than-ideal Option 3 while your S.O. and their friends end up doing something they probably didn't want to do in the first place. If any of you are ever tempted to do this, do yourself a favor and AT LEAST get the consent of all concerned S.O.s to participate in the activity you've planned for them. Much resentment will be avoided if you do so.
Option 5: Show up at their front door expecting to be entertained. Unless you're really, truly BFFs, so close that you would consider adopting each other and would take bullets for each other,
this approach shouldn't have survived beyond grade school. It was tacky back then and it hasn't gotten any better in the meantime. You're making yourself an annoying door-to-door salesman with nothing to sell. Calling with anything less than 24 hours notice fits roughly into this category. And no, even if you've said, "We should hang out sometime," and they have agreed, that doesn't make this any better.
