If civil union is effectively available to gays then they have a choice to be civially united, or not. But why should gays insist that "marriage" must legally apply to everybody who is civilly united or otherwise, whether or not he or she wants it to?
For the fourth time, every state that has a ban on gay marriage also has a ban on all forms of union that even remotely resemble marriage, and that includes your so-called civil union.
Why do gay people insist on marriage? Because they have been backed into a corner by all these laws that not only ban gay marriage but also any form of union that resemble marriage.