My final straw with my ex was that, during a discussion in the car on the way somewhere, he admitted that he didn't want children.
I had a MC a few years before, and everything was up in the air. I hadn't yet told him I was pregnant when I miscarried (he didn't want children so I was terrified of telling him - I hadn't known for long though), but it made me realise that I DID absolutely want children so I told him and we ended up breaking up because I said I couldn't be with a man who didn't want them.
A few months later he said he'd had time to think and he'd grown up and realised that a family WAS what he wanted etc. He knew I didn't want kids there and then (the first pregnancy was a mistake with my pill - my fault) so he knew he'd have a few years, I'm guessing.
Anyway I told him I was ready to try and he admitted he'd only said he wanted kids because it was what I wanted to hear and he wanted me back.
So there was me, another 4 years of my life wasted when I could have met someone lovely who genuinely wanted to start a family with me.
That was the end. I thought it was quite a cruel thing to lie about, really.