I fly for whatever side is available in every sim I've ever had. I've flown as Japan, Germany, US, Russia, UK, France, Australia, Canada, India, Israel, Pakistan, China, Italy, Spain, Greece, Turkey, Georgia, Ukraine...I can't even list them all.
I don't care who I'm flying for as long as it's accurate. I don't want to fly a Russian F-15 or an American Su-27, but I'll fly Finnish F/A-18s against Greek Mirage 2000s or French F-8Ns with no trouble.
Frankly, I can't comprehend how people can have a problem with flying for "the other side"...because it's not real! At no point ever in any sim have I been unaware of the fact that it's all fake, so all I think about is "shooting down the guy/AI trying to shoot me down." I never think "I have to kill that Chinaman" or "I'm about to shoot down a US Marine"...because I'm not.