Not supporting same sex marriage is a matter for the individual conscience as long as the person holding that belief does not actively campaign for it to be banned for others. There are various reasons why someone might oppose it- not even all gay men and lesbians supported it.
Indeed so. I was largely indifferent to the concept (I am civil partnered and wept for joy the day Blair's govt announced they were bringing in the CP bill), because I see marriage as something that straight people do. My partner and I do not refer to each other as 'wife' and cringe if anyone suggests that is what we are to each other. To us, a "wife" is a woman married to a man.
We did not, obviously, opt to 're-brand' our civil partnership as a 'marriage' as some others did and have no plans to do so.