I found myself in this position nearly 4yrs ago.
exH didn't want anymore, he was adamant, he hadn't had the snip yet, we (At the time may have changed?) lived in one of the few areas in the UK where it's not automatically availabe on the NHS - we couldn't afford the money,
I always wanted more, but had "agreed" with just 2 children, but always had in the back of my mind that once DS2 was older I would broach the subject again.
Anyhow, careless night, failed MAP and I got a BFP.
exH's first response was
"what are the options - I told him keep it or terminate"
"Terminate then" he said.
I was - I knew he didn't want anymore but he'd always (afaik) been anti=abortion except in certain circumstances (and a careless night with his wife wasn't one of them.....).
Anyhow, I thought about it, posted on here, and told him I couldn't do it. I knew deep down that if I went along it I would forever resent him and that if our marriage survived the troubles we had already/were going through outside of the unplanned pg there was no way it would survive my resentment. He didn't accept easily, but he did agree to support me and the baby. Although it took a couple of months for him to come round and even be able to talk about it with me.
Sadly our marriage was already on rocky ground and although we clung on for 9 months after DS3 was born (and later tried again for a while) it wasn't to be.
However, he still loves DS3 (who's now 3) just as much as he does the others.........indeed for a while when DS3 was a baby he doted on DS3 so much I had to remind he had 2 older children !