Because they aren't a husband and wife.
A husband and wife is what a marriage is. Once you start giving other pairings the same rights that are conferred on a husband and wife, you've already lost the argument against making them husband and husband.
If they are the legal equivalent of husband and wife, on what grounds do you segregate them into a separate category called civil unions? On what grounds do you refrain from teaching their "lifestyle" to the kiddies in the public schools? After all, they're the legal equivalent of husband and wife. Are you going to tell the kids they're equal to husband and wife legally, but somehow not quite as good so we don't let them marry? How long could you get away with that? And how long would it be until threesomes demanded the same civil unions?
Because if we recognize them as a 'Marriage' then under the 'Equal Protection' clause, they could sue our churches to marry them. A church would decide whether to go to court, risk their tax exemption - or simply fold and allow the marriage to take place. Thereby, taking the sacriment of marriage and making a sacrilege out of it. We both know that some gay people have this as their goal. A 'Civil Union' confirs the legality they demand, while protecting the sacriment we consider ordained by God.
As for their lifestyle, it's already being done. As for what to tell the children, that's up to you. Our kids and grandkids are taught that homosexuality is an abombination unto God. People commit all kinds of acts against God - and for those acts, they are accountable unto God.