With that said, I think it's bizarre how some of the commenters here are vilifying your management. I don't know the specifics of your company, but you can't run a startup at crunch time with devs putting in 7.5 hour days. Even at Google, which I considered extremely cushy and laid-back, there was the overt expectation -- repeatedly referenced in internal literature -- that engineer-weeks were about 50 hours long.
I think it's great that you've identified the level of work-life balance that will work for you and are standing your ground. But you need to understand that working 7.5 hours days is not some sort of universal human right. In the (near) future when you search for jobs, you need to communicate that preference up front and make sure that management is on board with it rather than taking for granted that they will be.