I do not know about other liberals, but I do not think that heterosexual marriage is 'special' in any way. All I am saying is that there are two different aspects of ANY relationship, homo- or heterosexual alike): the legal/institutional aspect that deals with property rights and contractual obligations and the moral/subjective/spiritual aspect which pertain to individual perceptions, values, feelings, emotions, etc. The former one is called the civil union and is already a pre-condition to a legally recognized marriage. Therefore, such civil unions should also be available to everyone regardless of their sexual orientation, skin color etc. Not allowing same sex couple to enter such a legally recognized union is equivalent to miscegenation laws in apartheid South Africa.
But beyond such legally recognized union, every relationship is subject to individual moral preferences and interpretations, and there is little that law can do about it. One can abolish miscegenation laws, but that will not prevent some people from not recognizing an inter-racial marriage as a "real marriage." Likewise, in my old country, many people did not recognize civil unions (the only legally recognized form of marriage) unless the couple was married in a church (which is why my wife #1 insisted on a church wedding despite my objections). But the fact that some, or even most, people do not recognize such a union as "marriage" does not mean that it is not legally recognized and binding. Ditto for same sex unions/marriages.
I do not understand why this basic distinction between law and morality is so difficult to grasp.
Wojtek