@zylya and @nycindie: I am pretty sure he is referring to looking at other women, as in, they pass through his field of vision and he thinks they are sexy. Marbit, please correct me if I am wrong. This is a real issue that (at least some) mono people have to deal with. Given the fairy tale version of romance, and the fact that women are not visually aroused nearly as much as men are, some women actually expect that their man will 'only have eyes for them', and will stop finding other women sexy. To them, comments like 'I'm married not dead!' are crass and offensive, and the people making them must not truly love their partner as much as they claim.
I am a scientist, and English is my native language, so the sources I speak of might or might not help. The main idea is that when you see something, it is processed two different ways in parallel. The first and fastest is emotional, responsible for fear, arousal, etc, and is why you duck when you see a fast moving object in the corner of your eye without stopping to think about whether or not it is even going to hit you. The second, and slower, is the conscious recognition and thought about objects. These different types of processing actually take place in (largely) different parts of the brain; the amygdala (emotional) and the visual cortex (object recognition).
The basic idea that you are looking for is not easy to find on wikipedia, but is so well understood and accepted in neuro-science that it is also difficult to find any scholarly articles; it is just an assumed fact. Still, if you are so inclined, here is some reading for you. They deal with, and talk about, the needed information. You might start with the summary and conclusion:
For a non-scientific reading try:
This is all good for your understanding. You need to know that you are normal and good just as you are. The last one may be good for your GF as well, since it is from a female perspective. Still, when discussing (or arguing) with her I would focus much more on emotional needs that scientific debates.
When she asks if you have looked at other women, I wouldn't lie. I have regularly lied to my wife, telling her what she wants to hear to avoid a fight. It always ends up worse in the end. We end up fighting later anyways, and it makes her trust me less. I have made a huge effort in our marriage to stay honest, especially if it means we will have to fight over it. In this case especially be honest, because if you lie to her and tell her you have not looked at any other women you are feeding her delusion that it is possible for you to not look at other women. You have to pull her out of her fantasy world!
I am here for you!