Social psych's an awesome topic...
Some circles call this 'social proof'... as if you're taking the word of someone else. Something to do with trusting other peoples' judgement, maybe?
Another perpective (You'll have to excuse the one-sided perspective, girls...) is that while a guy has a girlfriend, he feels more confident and happy with himself. Girls, in my experience, are very attracted to levels of confidence, even up to the extent of cockiness (as long as there's equal parts of humour in the mix, too).
So, one could assume that when you're with your s/o, you feel more confident than when you're not, and that comes out in subtle ways -- your stride, the way you make eye contact with her and other guys, how you hold yourself... overall, the image you project..
Thoughts?
-Steve.