Every woman's different, whether you're mysterious about who you went with or flat out tell her "You're the only one I want." there are too possible reactions. One, they pull closer to you, because they think you're out with someone else and they might lose you, or they use that to justify their leaving.
If you take the "You're the only one I want." approach, they can pull closer because it makes them realize how much you care about them, or they can pull away because they know you're always a good backup then.
