If I were home when they came, I would have asked about it. I don't necessarily presume that young canvassers agree with their church, but it played such a pivotal role in passing the gay marriage ban, and here they are canvassing for converts, so they're at least acting in support of it. Mormon canvassers are, as far as I can tell, always polite, so I'd be polite. I'd also make it clear that I believe their church is a terrible influence on the world and I actively advise my friends to avoid it.
If Mormons come to your door, remember to ask them about prop 8? I'm curious to hear what they say.