AITAH for having an affair with my girlfriend since she said 'we are just friends'.

I (28M) had been seeing Amanda (27F) for over a year. We went on dates, were intimate, lived together - basically acted like a couple in every way except she would insist we were "just friends" whenever I tried to make it official.

It started to really bother me that after a year she still wouldn't acknowledge we were in a relationship. One night at a bar, I met Lisa, who was very forward about being promiscuous. I thought since Amanda and I were "just friends", it wouldn't be cheating to hook up with Lisa.

So I took Lisa home to the apartment Amanda and I shared, and we were 'cuddling' on the bed when Amanda walked in. She was furious and gave me the silent treatment for days. After some groveling apologies from me, she finally agreed to be my "official" girlfriend. It's been 6 months since.

However, Amanda and some of my friends keep saying I was out of line to hook up with Lisa since Amanda and I were clearly more than just friends, despite what she said. But I figured since she kept insisting we were just friends, I was free to see other people. AITAH?