Is it possible that he is worried that your daughters will think he is trying "to be Daddy"? If they are grown and living away from home and the two of you move to a different home, then he will be one of the adults of the house instead of "he married our mom and moved into OUR home". Have you dicussed how life would be or how your daughters feel about you marrying this man? Do they like having him around? Do they treat him like an authority figure or at least with respect?
Wait, does he already live with you? Do any or all of your girls live with you? Is he still in another home? Are your daughters moving out as they graduate high school, or are they living with you through college and "planning" on moving out after college?
You WILL always be Mom, but the way families work when the kids move out is different than when they were under Mom's roof, or at least they should be. If he does not live with you, he may be hoping to have an extended honeymoon and not have to worry about being fully dressed every time he exits the bedroom. Right now, at least one of your girls is still in high school, I'm guessing. In this situation, he needs to be mindful of them being there and not running to the fridge in his civvies (or less). He may be worried the girls will "hear y'all" through the walls. You cannot be running off on a month-long vacation with a minor at home... it's not fair to leave her in someone else's care for that long.
If the girls are out on their own then there won't be discipline issues such as curfew, talking back to you, etc. If your daughters say something rude to you as minorts, he may be worried he'd make things worse by sticking up for you or defending you (it happens in some families), even if it's just a low-key "respect your mom".
Also, you say "I'll always be *the* parent". He may already realise that you don't want him to discipline or reprimand your daughters in any shape or form. That won't work in most families. If the girls figure this out, they could make life hell for y'all if they didn't want you two to get married.
I'm just brainstorming here :) I would definitely discuss this with him very intensely and I would find out how your daughters feel about it, as well.
Good luck!