Doesn't NI require a double majority to pass bills (a majority of Catholic and a majority of Protestant MLAs?)
Not on every vote. This isn't one that would automatically trigger a double vote, and even then the support level required is 60% overall plus 40% of both the Unionists and Nationalists. Conceivably, one could be called for, but even then, that probably only delays adoption of SSM by a decade at the most assuming neither the courts nor Westminster intervene in the interim.
This Assembly has voted on Marriage Equality motions in 2012, 2013, 2014, and most recently just last month.
The DUP did raise a petition of concern regarding the Marriage Equality Motion on three of those occasions - which forces a cross-community vote. They (nor any assembly party) have no qualms using the measure whenever it suits.
On all votes the Nationalist bloc voted absolutely in favour - with 1 abstention (or strictly speaking a vote both in favour and against) by Alban Maginness (SDLP, N Belfast) on the 2012 vote. (Maginness has absented himself from the subsequent votes.) The Nationalist bloc is inarguably solidly behind marriage equality.
Of the Unionist bloc - the near defunct NI21 (then 2 MLAs, now 1) are in favour. Claire Sugden (independent unionist) voted in favour in 2015 (she's a new MLA since 2014, so wasn't present for the previous votes). Only 2 other unionists voted in favour of either motion. Michael Copeland (UUP, E Belfast) and Danny Kinahan (UUP, S Antrim) have voted in favour 3 times (Kinahan once voted against).
The remaining Unionists, including the entirety of the DUP (38 MLAs), the UUP (with the exception of the two mentioned above, 11 MLAs), TUV (1), UKIP (1) have all consistently voted against the motions.
(Of the Others, the Green votes in favour consistently - and indeed was the originator of the 2012 motion; the Alliance Party appear a bit more muddled. Of their 8 MLAs, 5 appear generally in favour, the remaining 3 would appear to prefer abstentions or to vote against.)
To pass, 40% of Unionists will need to vote in favour. At best it seems there are 5 of 56 Unionists (9%) in favour. One of them, Kinahan won't be in the Assembly after next years elections as he's now an MP. Neither McCrea, nor McCallister (of, and formerly of NI21 respectively), nor Sugden has a safe seat.
On that basis, it's entirely possible (and maybe even likely) that the number of Unionist votes in favour of marriage equality will fall in the next Assembly. That Assembly should last until 2021. I'll hold off on prognosticating beyond that - suffice to note that it's a substantial swing required, and the views of members of the DUP hierarchy
on gay issues are evidently not softening very quickly.
I do believe that is a softness to a couple of the DUP and UUP no votes - but I think any expectation that the Unionist bloc will swing sufficiently in favour of marriage equality to allow it to come into force in the short-medium term is, unfortunately, rather unlikely.