Eek, I'm in the same situation. I'm a straight female, dating an FTM. I also found it a little strange looking in the forums here and not seeing another situation like mine.
Luckily my boyfriend passed very well even before T, so it wasn't much of an issue in public. I've introduced him to a few of my friends, and gotten the occational confused pause when they meet him and hear his voice, lol. I've found that, among my friends anyways, they accept him as male. If they have suspicions, they keep them to themselves, and they have never brought it up to me. In the encounters I've witness with trans or andro people, strangers are just looking for some sort of sign as to which gender they should classify a person as. I'm sure to get the pronouns out there, when possible, and people seem to accept it.
As for being concerned about how people see your relationship, I still deal with that from time to time. I still sometimes find it hard to see this relationship as normal myself, but we're getting through it together =) and it's been almost a year now.
Sorry I don't have any answers for you or your girlfriend. I just wanted to write something so you can see you aren't alone =)
Best of luck!