I'm not your girlfriend?

So my ex and I have been hanging out lately. We've been getting together at night after he gets off work and smoking a little bit of pot and then just talking or watching movies. Well last night I bought us Mexican food and we went to his friends' house (Cate and Cody). They had gone camping and left him the keys because he's planning on moving into their spare bedroom to help them with rent. So we went over to their place since they have a bigger tv. I was standing in the living room packing a bowl of weed and Cate's mom knocked on the back door. He went to go answer it. She just wanted to borrow a hammer, but she asked where her daughter was. He told her that they had gone camping and "I'm just over to borrow their tv. My girlfriend and I wanted to watch a movie. I am planning on moving in with them though so you'll probably see a lot more of us." I overheard him say this from the living room. We aren't dating?????? As far as I knew??? I thought we were just fucking around and hanging out. I didn't realize he thought I was his girlfriend?? I don't know what to do now because I do want to date him, but I also wanted for us to just hang out for a while until I was certain that it was what we both wanted. His friends have also been asking us if we wanted to do couple's photos with them and like???? We aren't together??? I don't know what to do about this because I don't want to hurt his feelings but I also wasn't expecting this and I had no idea he thought we were dating.