I think it would truly depend on the person making the initial eye contact, and on the person who receives the look, and the girlfriend/boyfriend. Everyone is different, so I would think there is no way to set this in stone.
Just eye contact cannot be considered flirting. I think it has to have some body language to back it up, like a come-hither smile. And I have to say, my wife and I both do it all the time. We tell one another, and we know it would never go anywhere because that just is not what we want.
On the other end of that, my wife came home one day and said, "By the way, if the fax phone rings and a guy asks for me just tell him he has the wrong number!" When it happened, I did as she asked and he never called back. She did not want to be blatantly rude to the guy, yet she has a hard time saying no to anything. It's all good.
This is an excellent question. Thank you for asking it.