So, as I heard it explained, and you can correct me if I'm wrong, the big swing politically is that marriages have tax benefits that civil unions do not. The government wants to sponsor marriage and families as they are beneficial to society. You hear about that sort of thing a lot when you discuss education. The single biggest factor affecting grades, drop out rates, etc, is family. The governement acknowledges this and trys to encourage families through tax breaks and stuff. It's not all that different than encouraging home ownership by letting you deduct the interest you pay on your mortgage. The question that is yet to be answered is whether you see those same sorts of benefits with same sex couples. Clearly you can't bring forth children but then you could go into adoption. Anyway, it's not really a question that I personally care enough to try to answer but that's the reason why it's not as simple as "mind your own business".
WE ALREADY OPERATE IN A MIND YOUR OWN BUSINESS SOCIETY. IF YOU WANT TO GET MARRIED TO YOUR SAME SEX PARTNER YOU CAN. NOBODY WILL STOP YOU.
The issue is over government endorsement of marriage and stuff like that. If you guys want to go on about that then have at it.