Because we live in a nation which prides itself on equality for all of it's residents (of course we all know that is far from reality), myself and many Christians that I know have no problem with homosexual couples filing jointly on their taxes, carrying the other on their insurance, allowing their partner to be their beneficiary, etc. All of this can be accomplished through Civil Unions.
However, for most homosexuals, gay marriage is not about equal rights. It's about using the government to force others to accept something that their religion teaches is morally unacceptable. In doing so, the homosexual sexual agenda is pressuring the government to violate the first amendment right to freedom of religion for many Americans regardless of what religion they believe. This must never be allowed which is why Civil Unions are the only way that homosexuals will get equal rights without violating the rights of others.
Pretty cut & dry....