Oh op, I really feel for you. You’re deep in the trenches of young children and your team mate has abandoned you.
He certainly doesn’t sound like he’s coping well and instead of pulling together is pulling away which is awful for you and putting more pressure on. So you’re in a viscous circle.
It also sounds like he has developed what sounds like a porn compulsion / addiction - this will be why he’s not wanting sex, or saying he doesn’t fancy you. This is not about you - this is his problem and he really needs help to address it. At the point he’s wanking in the kitchen with small children in the house, I’d say he’s in the grip of addiction. He’d need to recognise this in order to get treatment. You can support him, but you can’t cure him of this.
For what it’s worth, many people would consider OF subscriptions cheating - don’t let him trample your boundaries if this is the case.
It sounds like you either need an intervention as a couple, or you need to cut your losses and start afresh as co-parents.
I know how terrifying the thought of parenting alone 3 under 4 is, but if he’s away a lot you’re already doing this and I bet my bottom dollar you are stronger and more able to cope than you can possibly imagine. Think hard about what it could look like if you were to separate; how would you like it to work? What would your preferred share of childcare be? Housing? Work? Support network etc.
Speak to a solicitor so that you can build this new reality on facts. Then present those facts to him. It might serve as a wake up call when he sees that walking away won’t necessarily make his life easier. Working away might not be possible for example if he’s sharing parenting responsibilities- how would he feel about that?
I’m sure it’s tempting but I urge you, don’t beg him not to leave - he’s showing you enough disrespect as it is and begging never made anyone stay for the right reasons. However I would strongly urge you suggest couples counselling - you could put it to him that you want couples counselling to explore both trying to make it work and separating well.
You may find quite quickly that life without someone so unable to work with you, to show you the respect and support you deserve and who abandons you at the most difficult time in a marriage is so so much better. Hard, but more peaceful. 💕