you'll probably be able to do so. While the last entry in the series blocked Xbox Live's Party Chat in many of the game modes, Modern Warfare 3 will apparently be much more forgiving.
"The only place where Party Chat should be blocked is Search & Destroy," says Bowling, since that's the only mode where players are meant to have only one life per attack, and allowing dead players to chat with live ones might create an unfair advantage. For all of the other modes, "it's about giving options. We will have places where competitive guys go where you might have restrictive voice chat. And then you have an option to maybe play that same mode without the same restrictions if that's the type of player you are."
So, good news! You and your girlfriend can have some private time together to say lovey-dovey things in each others' ears, even while you gat some noobs. How romantic.