That means that the guy is already dating another girl and so can't take you with him so I think you should not mind this as he didn't lie to you and went for coffee he just told the truth and denied so nothing to get sad about! :)
What does it mean when a guy says he's sorta seeing someone? I asked a guy out for coffee on facebook, but he said he's sorta seeing someone and he's really sorry? What does that mean?
He obviously doesn't want to hurt your feelings so that's a sign of him caring, but he seeing someone else he probably likes her more, but at least he showed you respect by telling you.