Hmm, it is a tough one - at this point your baby is completely perfect, and it's hard not to become really ott at the thought of something 'spoiling' that perfection (for both of you iykwim)
Compromise seems in order, and seeing as he has already given the dcs his surname, it would only be right that it's his turn to do so, but if he's having the same weird reaction to the name as I do, it will be almost panic inducing for him.
I would honestly shelf talking about the whole idea until after the baby comes. You still have six weeks to register iirc.
You could even suggest it as a 'second' middle name - if you mention nothing else about it till then - I certainly wouldn't waste time arguing over a theoretical at this point. You 'feel' strongly' it's a girl, but it may not be...
If you have left it be, given him some time t reflect on his horridness (and I'd have words about his 'delivery') then you can bring it up at a more appropriate time and prepare your argument. You've said some fair enough stuff about why the name is important to you, and I think he should respect that.
Whilst I really dislike the name,
I feel your reasoning is totally sound.
Fwiw, my ex chose the names for my dts and I wasn't overly thrilled, but I went along with it as they were having my surname. Then at the last minute my dad (who I no longer see as a massive nob - another thread entirely) put serious pressure on me to give them ex's surname too!
we ended up giving them a double barrel surname, and ex got to choose the first and second names.
Fast forward nine years, and I am not fussed in the slightest. They are who they are, and I love them. Possibly your dp will be able to 'get over' the name Margaret being tucked in somewhere, and I feel he owes it to you as you gave them your surname.
Phew!