Well again to use the current UK government as an example, both Lords reform and streamlining of boundry changes have failed to happen, despite supposed agreement between the parties.
As for gay marriage, Cameron didn't need to push for it, the Lib Dems never made it any kind of make or break issue, and it certainly wasn't in the coalition agreement. I think Cameron genuinely thinks its a step forward, as do a lot of Convervatives (yes a majority voted against it, but the numbers were close to 50/50, and they definitely wouldn't have been like that just a few years ago).
And turning your own example about abortion on its head, if Labour had a decent majority they could just enact the legislation themselves anyway?
So I refute right back at ya