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.
If we're going to allow that, we should have civil unions for threesomes and foursomes. And we should have civil unions for people who aren't even sexually involved with one another. They just happen to room together and claim to be fond of one another, so why not give them the benefits of married couples? If it's what they desire, shouldn't we give it to them? After all, that seems to be the criterion these days.
The only solution to the public schools propagandizing the kids on homosexuality is for society to become healthy and sane again, and that means coming right out and saying that homosexual relationships are unnatural and, to be blunt, are no damned good. Saying they're just as good as marriage, but are slightly different so we give them civil unions instead of marriages, is just a short term tactic the left is using to lure some conservatives in for the kill. Civil unions will never last, for obvious reasons. They're "separate but equal", and liberals will never allow that the stand in the long run.
Once you concede that homosexual couples deserve all the rights and privileges of marriage except the name, you've lost the battle and same-sex "marriage" will eventually become law.