So... an update
Well, we went ahead and signed a lease. We ended up with a four bedroom house, just under 2000 sq ft. A little smaller than we wanted, but a better location (shorter commutes for everyone!). And were able to find a situation that if someone moved out, the remaining three could still afford it. We're moving in on August 1 with a one year lease.
No one has raised any eyebrows about it being weird once we mention that it's only $1495 for 4 people (my husband and I are already spending ~$800 for our 2 bedroom apartment and husband's girlfriend is spending ~$650 for her one bedroom). Plus, we managed to find a place with a big front room (20 x 24') with wood floors that is just perfect for dance parties!
We have all agreed that even if things go south romantically, we can still be adults and good roommates. We are all bound and determined to be sane and reasonable. We can all stay friends. I will admit that I asked my boyfriend not to sleep with my husband's girlfriend, cause that would weird me out. But everyone is free to cuddle as much as they want.
We are all committed to keeping the place nice. We really want an adult space that is tasteful and high functioning. We are acknowledging the fact that it will be difficult to share space. I haven't shared a bathroom in over 10 years (yep, my husband and I have a 2 bed 2 bath apartment and the master bathroom is *mine*) and I'm kinda nervous about it. My husband's girlfriend is an artist and has a lot of supplies. We're turning the fourth bedroom into a guest room/office/craft room so that all of that mess can be behind a door and I don't have to see it.
We're going to try to make this work. I see it as something that we try for a year and then maybe readjust. My husband thinks this could last for 3-4 years. We'll see. We're going to try to prepare as best we can for the worse, but hopefully it will be amazing!