Our marriage guidance counsellor told me that approx 75% of the men she sees are like children, at least where carrying the mental and emotional load of family life are concerned - and often the labour, depending on how they’ve been socialised/how much of a knob they are/how amenable to direction (aka control) they are. My own husband falls into this category and I didn’t fully realise it until (much wanted) baby number three came along - I really do think breaching the two child ceiling exponentially increases the parenting load.
To be honest, it didn’t drive me to the point of seriously wanting to split until life threw so much shit at us that I became incredibly resentful at having to direct (control) him, and I could no longer do it with humour (jokes about extended poo-cations, singing ‘I am a passenger’ at any signs of irritation from him at being ‘nagged’ etc etc. I am controlling. I wish I didn’t have to be. I didn’t have the issue of him going out for more nights/weekend days than was fair or reasonable, but if I had, I would have had no qualms in objecting and if he’d continued, yes, it would have affected my mood and I’m sure I would be cross, irritable, sullen with him. That’s not control - that’s a reaction to someone taking the piss/taking me for granted.
We’re still together, but if he found someone else and decided to leave, I have absolutely no doubt that one explanation for it would/could be that I am controlling, just as if decided to leave, the reason would be that I can’t go on any longer without a true partner, that I’m carrying too much. Both of us would be telling a version of the truth if this happened, but the real truth would be that we just didn’t want to continue with the marriage because it was taking more from us than we were willing to give.
I suspect your ex didn’t have that much to give in the first place, in terms of how willing he was to prioritise the family rather than his own wants/needs, and that the twins tipped the balance, largely because you probably needed to ask for more from him than you had hitherto with your first two children, where you had a decent age gap, and could probably get on with things without much help. He’s left because the marriage and family required him to be a better man than he can (or is willing) to be.