This wasn't a roommate/gf situation, but my sophomore year in college, I shared a 2-bedroom apartment with two of my friends. I was in the shared bedroom. I don't remember the exact split, but it was between 33/33/33 and 50/25/25. If the rent was, say, $600, my friend with the solo room paid maybe $250 and the two of us sharing a room paid $175. Something like that.

Utilities were split evenly three ways.

We were good friends and not nits, so it was pretty easy to agree to the numbers, whatever they were.

why not just move out and live with your gf?

What are you talking about? Assuming it's a 1200 square foot place with equally sized bedrooms at say 150sq ft each, that means it's a $750/$450 split, which is a number you yourself already proposed.

Assume: $1200 rent, 1200sq ft place, two equally sized bedrooms of 150sq ft

Communal: 900 sq ft
Bedrooms: 150 sq ft x2

Split:
Person 1 (bf): 300 + 75 = $375
Person 2 (gf): 300 + 75 = $375
Person 3 (single): 300 + 150 = $450

P1+P2 = $750
P3 = $450

He's saying that 1/3 + 1/6 = 1/2, and it doesn't sit particularly well with him.

Chopstick, I don't think communal space in any 2br apt is 6x the size of a bedroom.

With 150sq bedrooms, the communal area is probably closer 300-400 sq ft

After a GIS for "2br 1a layout sq ft", estimating 2 bedrooms taking up 25% of total space was definitely wrong on my part. It looks more like 33-40% is standard. I wouldn't go as high as 50% though.

So if it's on the high end at 40%, numbers look like this:

Assume: $1200 rent, 1200sq ft place, two equally sized bedrooms of 240sq ft

Communal: 720 sq ft
Bedrooms: 240 sq ft x2

Split:
Person 1 (bf): 240 + 120 = $360
Person 2 (gf): 240 + 120 = $360
Person 3 (single): 240 + 240 = $480

P1+P2 = $720
P3 = $480


Still not very far off from the $750/$450.

I did this with my current GF and a good friend for around 16 months, we worked it out as:

$2100 a month rent

Me - $800, GF - $500
Friend - $800

Bills split 3 ways, no exceptions on "I don't use cable, I don't have 2 hr showers, I don't want to pay for UFC PPV" all that rubbish, it was always split 3 ways.

In your example, make sure that the couple each pay, as there would be no way this scenario works if the GF doesn't contribute. I've had friends who did this, where the BF was paying the full amount, and the GF didn't work or contribute, and it made their life hell, when it came time to pay for shared utilities and Cable bills.

On a funny note, my roommate cleaned up with my current Gf's friends (5+) and is currently dating one of them, so yeah make sure if you are living with a couple, the girl has good looking friends.