An unmarried (in the eyes of the state) homosexual couple are able, through domestic partnership provisions (state, company specific), to extend employer based health benefits to their partner.

They don't have the marriage option to begin with, you're right. Marriage doesn't even enter into that picture since alternate provisions have been or are being created to cover said individuals and situations.

Why must I or why should I get married just to receive benefits if homosexual couples, denied full marriage rights to begin with, can get benefits fully extended?

I'm trying not to cloud the overall situation and I'm certainly not coming down on homosexual couples or trying to lay blame on them and I hope everybody reading this understand that.