You do know labor was going to change this anyway right?
True. But I'm pretty sure Labor were hoping "No" would win.
Gay marriage has been a political tool for a long time. I have to give props to Turnbull for a very smart political tool. Both sides knew that they would get a big boost by passing gay marriage legislation. There has been about 13 same-sex marriage bills put forward over the past decade and they've all been voted down for one reason. No one wants the other side to get the credit.
If Turnbull put forward the gay marriage bill without the plebiscite then it would be voted down by Labor so that Labor could put the bill forward when they took power, and that bill would be voted down by Liberals. It's a never ending cycle.
Now that we've had the plebiscite it has become very heavily focused and very real. Liberals can now put forward the bill knowing that if Labor votes against it then Labor will have no chance of winning the next election. So Labor are forced to concede defeat and support the bill. If the plebiscite came back as a "No" win then Liberals wouldn't put forward a bill and Labor would run their campaign of "Liberals are evil, we're the heroes that'll give you same-sex marriage"
With this move Turnbull may have saved the Liberals from defeat at the next election. Of course Liberals could still screw themselves by ousting Turnbull which will push everyone to vote Labor and Nationals.