Yeah, I get that. You're confused. You don't understand it. I don't think anything we're going to say is going to spark enlightenment. It's about compartmentalization between legal and religious/social.
But by insisting that they're pretending because you find it "strange", you are projecting negative behaviors on people that they are not doing because that's what you would be doing if you did the same thing due to YOUR beliefs.
Many people (I won't say all because I don't pretend to know ever couples' beliefs and circumstances) are not pretending at all. It's what they believe. You are actually being very insulting to them by insisting on calling it pretending. So, on the one hand, you're saying "I respect people believe differently than me" and in the next breath, you're saying, "But if you do believe differently, you're a big fat fibber." And no, not you're saying that word for word and that's not a direct quote, but your word choices can speak VOLUMES. Telling people they're pretending is either insulting or condescending. Most likely both.
And no, I am not defending this because it's something I did. I got married in a court house and was darn happy to do so! But I also feel we should be open to celebrating others' beliefs and happines despite what we may believe.