IMO, an anti-religion isn't okay. You can be against a religion. But founding a BM religion as an anti-religion is obviously metagaming. It's not an SMA issue, it's a "You're abusing the feature" issue.
You should interpret "anti-religion" more as "replacement religion." One dedicated to destroying the other, yet ready to assume the power and functionality the former had. It may have a similar god/pantheon, beliefs, traditions, etc... This is perfectly SMA and obviously not metagaming.
