I might be wrong, but my understanding is that "ma femme" is wife, because she's yours. But une femme is a woman. If you wanted to use wife in this sentence you would use épouse. I had major issues with the words amie/copaine and read a lot about it. I came to the conclusion that it is all context, but "ma amie" and "ma copaine" both imply girlfriend because of the ownership, and if you want to talk about someone who is just a friend you would say "une amie". "mes amies" complicates things, because it would normally be understood as two friends, not two girlfriends, but you don't really know!