Mark Patience, hi
In your situation, I would have just exited the game immediately. Portal won't run away, while your girlfriend can. Heck, you can even use the experience gathered so far to blaze through the earlier parts of Portal with new strategies later on!
There's not much that can be done, I'm afraid. An incredible majority of people grow up playing games as children, then have to take on responsibilities (relationship, marriage, family, etc) and games are discarded in favour of "serious" activities (i.e. playing the serious game of "life" where you have one life, zero continues and a continually ramping difficulty level
.
As such, when hearing the words "video game" (emphasis on "game"), that large majority automatically and strongly associates them with "childhood", and thus they experience a mental conflict: they clearly see an adult person partaking in a "game", which they believe is reserved solely for children or family parties when everyone had something to drink. This association is reinforced by the fact that it's always the children that ask for the video games. Everywhere, people are getting video games for their kids for christmas.
It is very difficut to get rid of this association, since most people don't have much of a choice in life - they HAVE to take on responsibilities and they don't have the luxury of "gaming". The word "game" simply has that strong an effect. All we can do is be patient and wait for the masses to change. For that to happen, new associations with the words "video game" have to be made. For example, maybe people will start to see it as a "sport" (seeing TV reports on professional video gaming may be a good start), or "hobby", even. Or, video games transcend their generally low quality storytelling and give the adult gamer some real adult content other than cheap gore, sex, cursing and violence. Something with the impact of "Animal Farm", say, but in video game form.
But yeah, right now, a "video game" is for "the kids". Until then, we are in the privileged position of witnessing just how the public opinion on video games will evolve over the coming years. Let's hope it ends up in the right place.