The idea is about legal equality. It sounds subtle, but is really very important.
All this talk about we-really ought to just have something special just for us does not treat that de jure is absolute not de facto equality. Many people do not understand what a domestic partnership is, even if it is a de jure equivalent.
As jayhawk said, separate is very much not equal. If you commit to someone your whole life, the difference will matter during some of the most critical parts of it, especially through death and bereavement.
If people are rejecting gay marriage because it is too conservative, they are giving the wrong reasons to oppose it. Society treats marriage as very important whether we like it or not, and domestic partnerships will forever relegate gays as less than equal. It's really too bad if some people don't want that to be true, because wishes don't change how other people perceive us. One cannot walk through life without interacting with the public, and therefore how the public and the government perceive one's personal relationships matter.
In the end, if there are gay couples who do not want marriage, that is fine, but do not make that choice for the whole community. Separate but equal does damage, and that's why our advocates have made marriage equality the cause celebre. Rejecting conservative institutions may be an important goal, but it is important to remember that damage to the LGBT community is done when that principle is not advocated for universally.