As PPs have said, being in a group photo at a wedding is a totally reasonable expectation, and I think anyone who refuses is being a bit self-centered and snowflakey.
That said, I totally take Venus's point about it being rude for the hosts to INSIST that people participate, if they're clearly uncomfortable. Requesting, or encouraging people to join in is fine. Calling individuals out by name in front of everyone, and demanding they participate isn't.
I experienced this myself at a wedding last year (and even wrote a thread on eHell about it). All the unmarried people were asked to come up on stage to play a dreadful game that involved randomly being partnered up (male-female) and popping a balloon between our bodies without using our hands. I initially refused (as did many others). But then the MC began calling people out by name, over the microphone. She was like a dog with a bone, and would not rest until most of the unmarried people had (reluctantly) agreed to participate. That, I believe, was rude.