have fun not playing with them
I absolutely do.
I'm personally not interested in MP anyway, but I will occasionally play if there's a Linux compatible game on offer, but I'll probably burn out after a few sessions anyway. For example, I used to play L4D and similar co-op games with friends, but I got bored after finishing the campaign, so I bailed when they were still playing together.
There are plenty of MP games out there that work on Linux, and I've played several of them with friends. The big MP games just don't interest me.
So it's less "stand up for Linux" and more "stand up for yourself." If you don't like a game your friends like, for whatever reason, feel free to just not play it. I recommend suggesting something else that does interest you. We got into a rut and it turned out my friends didn't really like the games we were playing anyway, so recommending something else really spiced up our gaming time. Try it out and see what works.
But if you really want to play something, I don't see a problem with firing up Windows from time to time. I have it installed on my system as well, I just haven't used it in a year or two, but it's there if I need it.