All of the hyphenated last names I've ever seen run at least 10+ letters, most last names are at least 5, so if you combine two...there you go.
I would really move to compromise on this, to be honest. Of course, as an adoptee, I think that validating one's heritage can be extremely important, and names can be even more important. You kind of were able to luck out on this, because when you were married, your DH had no attachement to a name.
Now he does. While it may mess up the absolute visual appeal of the name for you, for awhile...I think it would reflect your compassion and honoring to include him if you were to at least discuss this some more and offer compromises until you come closer to a consensus.
I guess you could stick to your guns about "well, you made an agreement before this came out, so I win". But in many, many relationships things change over time, and people end up compromising on "important things". If it wasn't difficult, it wouldn't be a true compromise and inclusion.
How much time do you have left to make a decision? It's true, if all this recently came out, he's likely to still be in the shock stage.
But as for WWID, to be honest I would hyphenate the last names of all the children. Of course, I don't go by the "woman rules no matter what since she gives birth" deal either. Presumably a loving, committed partner cares for the woman during her pregnancy and supported her in conception or contributed to it, and cares for everyone in the family during and afterwards.
I think you need to do some more talking about this. But I think it would be both compassionate and wise to at least attempt to reach some sort of compromise.