I am having similar issues with my guy friends. I am pre warning them that my DP may whip it out at any point in time to feed the baby and if they have issues with it I will just hang with them in 2 or so years when the baby is done eating lol.
That being said I think it is an american hang up that is ingrained in some people that breasts aren't for feeding babies. I have no problems with it and I am actually hoping someone is 'uninformed' enough to confront my DP while I am within ears reach.
I would stop accepting the side rooms (unless you are doing it to keep the peace and don't really mind the alone bonding time, that is up to you) and I would let the DP know that if he wants to walk a mile ahead that is his perogative. I wouldn't make excuses for it, but I doubt he will get comfortable with it unless you just don't make a deal about it and keep acting as though it is natural.
It may be so ingrained that he doesn't get over his hang ups, but if you wanna get snarky you could ask him to eat in another room after you make a 'fabulous' steak dinner because the way he eats his steak offends you. And repeat the process every time he makes a comment.
I don't 'really' recommend that, it's just something i'd do if I was the one breastfeeding.
I only have 1 friend so far that has said anything about the upcoming breast feeding events. And it happens to be my best friend, but if he thinks that I am going to make my DP go to another room (unless she wants to) he is sorely mistaken and it will be him, the uncomfortable one who will wind up vacating.
Love him to death, but I beleive that the lactivists have it right in just acting natural about it.