I think his insistence that his other two are yours is a sort of emotional blackmail, far from your understandable sadness doing that to him. He’s implying you don’t think his children are enough.
OP, it’s a different situation and it’s the only thing DH has ever said that made my jaw drop, he’s a wonderful, emotionally literate gem normally... But I had a devastating missed miscarriage when we first started ttc and on the way to hospital for surgery he said “I know it’s hard but remember you’ve got mine to share”. I was bleeding in a lift on the way to say a final goodbye to my beloved baby and it popped into his head that mentioning I could share his existing children would be a comfort to me. I mean, really. I love his kids to bits but I wanted my own fucking baby, the one I thought I was having! I told him once the dust had settled how breathtakingly shit and insensitive it was and he genuinely couldn’t remember as it was such an awful time and his head was shredded but he apologised and hasn’t been so bloody stupid since.
The struggles we had getting DD are one of the reasons she’ll be my only and I’m glad she’s got half siblings but the time it took us meant the age gap is much bigger than we’d planned and she will be an only child in practical terms in a few years which I’m sad about.
All this well you should have discussed it beforehand and you knew what you were getting into is absolute bollocks and you’re best off ignoring it, especially when you’ve repeatedly said you did discuss it. When it comes to making babies few things go to plan and there are infinite variables, compromises, surprises, unforeseen circumstances.
You accept his decision. You’re not trying to change his mind. All you’re asking is that he stops lying and asking you to pretend his children are actually yours.
If he dropped dead tomorrow you’d have no rights to ever see them again. If you got divorced tomorrow you might not get to see them again. The baying hordes on here insisting it’s not about blood and step kids are the same as having more of your own would be exactly the same people calling you out of order if you insisted on going to parents evening, if your husband gave you the second parent ticket at a school play, if you posted on Facebook referring to his kids as yours, if you disciplined your SC, if you made decisions about their clothes or had their hair cut.
You’d get a chorus of “they’re not your kids, they already have two parents, who do you think you are, you’re just dad’s wife, not mum, know your place.