Because - sadly, because I agree with you that they are wrong - over half of California opposed gay marriage in 2008 (and while things have improved somewhat, they remain far from where they should be).
They are wrong, but their position is a mainstream one. I'm not sure refusing to work with them is practical. (It might be in a small company, especially in ideologically uniform regions like the bay area, but not in general.)
Do you do background checks on people you might work with to see whether they hold such views? Or do you just avoid working with people about whom you found out by accident that they hold such views?
That is, are you actively making sure that none of the people around you hold the views you find unacceptable, or is it more of a don't ask don't tell kind of policy, where you don't care if people have unacceptable views as long as they don't mention them in your hearing?
We're not talking about disagreeing on tax policy here.