I understand completely how you feel, Sage,
I feel like my wife and I get to deal with all the mundane, often difficult, trying parts of a relationship, such as paying bills, sharing to household work, and just trying to get by. I feel like she can forget about all then when she is with M. It's unfortunate, since, he's a bachelor and definitely living the bachelor lifestyle, but if I let our house get so disgustingly filthy and cluttered she would have a fit! But she doesn't ever say anything about the state of his place to him, she just deals with it. On the plus side I think my wife and I have a much deeper emotional connection because we have had to deal with the hard stuff, the tough decisions, and all the mundane bits of life that get in the way of just having fun.
I'm not sure if I consider myself a mono-primary, but that's how things are at the moment nonetheless, so I know where you're coming from. My wife and M get to go on a weekend getaway while I'm stuck with the housework, the bills, and dealing with her work clients that want to change their appointments or add more time or what have you. Lucky for us we don't have kids yet, but if we do that will make things even more difficult. Sometimes its hard being the 'beast of burden' so to speak. You should probably just tell him how you feel about it, and let your frustrations be known. Sounds to me like you and your partner need to take some time just for the two of you once in a while, and you have to ask for what you want in life, right?
And so far as the type of love your husband and his gf have, you're probably right, they are in love in a bit of a fantasy world where they do not have the same responsibilities and he can just leave his responsibilities with you. If this bothers you, you need to tell him. You deserve some of that fantasy too, just as she deserves to see the piece of himself he leaves behind when he goes to see her. Being the meatloaf and potatoes in a relationship can often feel unfair, but remember that a person can't live on the desserts alone