In fact, that's the basis of the difference - the key argument. Is gay marriage/rights *civil* right or not? You could flip it around and question how a party can be more upholding of civil rights if they support abortion (as in - most Democrats support the abortion choice)
(That doesn't sum up my thoughts on things, though)
There's more to the issue of 'civil rights' than 'gay marriage' - and even many Democrats are opposed to gay marriage, Like President Obama.
Basically - it didn't factor into the equation for me when I chose 'both'