Outsider: Stop worrying about them, and focus on the other outsiders. Become friends with *them*. Don’t try to copy the clique-y ones to get back at them. Just use the opportunity you have to make new friends with the other ones who are open to it. Heck, those guys could be your friends, too. It doesn’t have to be divided into men in one group, women in the other. 

Also, like someone else said, they were friends already. They may not be doing it on purpose, they just have a history together. Sure, they could be more inclusive, but, like with the bachelorette party - why would anyone invite acquaintances to an event that’s meant to be for your closest friends? 

You can’t force a close bond with a group of people whose main thing - or only thing - in common is that their significant others are friends. If it happens naturally, great, if it doesn’t, oh well, don’t take it personally. Make friends with those you can - girl or guy!