I wouldn't worry too much about the excuse he gave for not seeing the girlfriend, the important point is that you needed him and he stayed with you.
I think you are quite right to be upset about him bringing his girlfriend over without discussing it with you especially as it was the first time. It seems to me that people have very different rules and expectations about things like this and I think it partly depends on where you live and how you have been brought up. Most people would frown on the arrangement that my husband and I have, but it works for us and we are happy with it so it is no one else's business. My husband wanted a rule that our home was just for us so we do not bring other partners to our home. At the time I think he just wanted to protect our family and I was happy to agree, but I would be willing to relax that rule in the future. We are quite private people anyway we wouldn't usually bring any visitors to our home without talking about it first. Different countries and different people have different traditions and expectations.
It's always hard at first, but you need to talk it through honestly with him rather than arguing. Just be direct and tell him that you need to talk because this isn't working for you at the moment. Be clear about what you need from him and listen to what he wants, be willing to compromise on the things that don't really matter too much, but if something is a big deal for you say so. Don't expect everything to be resolved straight away, it takes a long time become 'comfortable' with such a big change (or at least it did for me) but things do settle down in time. Just be open and honest and focus on being constructive rather than arguing.