The gay marriage matter was going to go that way sooner or later regardless. Look at younger voters of any party in the US (after all the overwhelming majority of slashdot readers are in the US) and you'll find increasing support for gay marriage even amongst those young voters who vote republican. I don't have the time to look through 1000+ comments in that article to see how many are viewing the matter through each possible prism but there is also a very heavy conservative-libertarian bend here, who will often be saying "let them do whatever as long as it doesn't increase my taxes" - and gay marriage certainly doesn't increase anyone's taxes.
While you're at it, see the posts about AGW.
The key part of that acronym is the letter A - for anthropogenic. There are occasionally posts here on climate change, but very exceptionally rarely do we see any that tie it directly to human activity. Furthermore if we look at the comment sections in any of those articles - anthropogenic or not - we see the true voice of slashdot readers bends heavily to the right.
And finally, notice that at least once a week we see an anti-Obama - or generally anti-democrat - article on the front page here. How often do we see an anti-GOP article? Almost never. And I challenge you to show me an article that ever made the front page that dared to challenge Ron Paul on anything, ever.
Personally, I find /. to be center to center-left, depending on the subject.
I'm curious to know what subject you think that would be. Certainly not politics, science, economics, constitutional liberty, crime, taxes, religion, birth control, or education.