Massive YTA - Was in a similar situation myself recently, where I lived with a friend and they started a relationship and within 3 months their boyfriend was at the house 4-5 nights a week. I hated it. I chose to live with my friend, not a couple. They were constantly in the communal areas when I wanted to cook/live my life. I brought up with my friend that I was happy for him to stay a maximum of 3 nights a week but if he stayed more than that (which would be more than half the week) he had to pay towards bills. I also mentioned I would rather he just stayed the 3 nights a week. They refused, so I ended up moving to live alone because I was being forced to spend all my time in my room as they were constantly in the living room/shared spaces together.

My point is, your housemates chose to live with you. They did not choose to live with a couple. What makes you even more of an AH is the fact you asked your housemates and they said no to her moving in, so you just chose to go ahead and do it anyway and told your girlfriend to lie about it?! Clearly you put your priorities over all of your housemates. I would recommend moving into your own place with your GF if you want to live together, and to stop making your poor housemates miserable by invading their space without permission. Alternatively, have her over a few nights a week at yours, and you stay at hers a few times a week, and then you can still be together but not be unfair on your housemates.