a girlfriend ... a boyfriend
Let's begin by having you define your terms. What
exactly do you mean by those words? And do the two of you agree
exactly on those definitions? Some possibilities:
- dating buddies -- we go out in public openly as a couple & have a lot of fun together & don't worry about more intimate things
- flirtations -- maybe up to hot chat & emailed photos of body parts
- occasional fuckbuddies -- mostly sex, not much social interaction
- FWB -- close emotionally &/or intellectually, maybe with sex sometimes but we don't stress about it
- strong secondary -- dozens of hours every week & two or three overnights
- steady partner -- even closer, & regular interaction in my home with partner & metamour(s)
- cohabitor -- living together
- marriage or equivalent
You cannot say something like "well, I don't know yet, we'd just have to see where it goes" because the reality is that the two of you already have your preconceived notions -- claiming otherwise is begging to get bit on the butt when incompatibilites become insurmountable.
IMO, waaaaay too many "poly" noobs begin from too many silly assumptions. A friend compared it to fishing: the reality of finding a potentially great relationship is like baitcasting -- you often get lots of weeds, trash fish, & the ones too small/big to keep, but sometimes there's a glorious strike that results in a trophy fish. Beginners, though, see the pictures but not all the preparation & waiting & the hours & days that went into getting that fish. They talk themselves into confusing SIMPLE with EASY, & decide all that needs be done is wade into the water with a spear, plaster anythig that swims within reach, & call it success. (Then come to a site like this to complain that the result was not at all like "what was promised."

)
It's great that the two of you have been able to discuss nonmonogamy. Neither of you is likely anywhere near ready to ease into it. An option: leap in blindly & trust each other enough to sort out the wreckage & learn from the experience, then try again. (It works for a few people.)
The middle path? I recommend that you two learn how to simply have close friends of the opposite sex -- NO romance, NO physical affection that you wouldn't share with a sibling, certainly NO sex. Concerts, movies, farmer's markets, whatever -- figure out how to present as a couple with NO NEED for it to "go further." Learn to do it without anyone getting their little feelings hurt or having panic attacks.