There are more than a few gay people who are using this as a wedge that will be progressively (pardon the pun) driven home to punish those who oppose homosexuality. These are not your nice gay coworkers and relatives. There is a militant gay faction. It is already illegal in Canada to say that homosexuality is a sin. You need to consider the militant gay endgame. Are there just a few of them who have no real power or are there enough of them to continue driving in the wedge by means of the legal system?
One could also ask, "Why would you trust someone to do a good job baking your wedding cake who didn't want to support and celebrate your marriage?"
There are radical factions of almost every group (religious, gay rights, environmentalist, political, etc). While they might actively push their agenda, that does not make them right. And it certainly does not mean you limit the rights of the larger group because you fear the subset.
Again, we must start from a position of equal protections for all and deal with these issues as they arise.