Depends on the relationship of that person, but generally speaking, yeah, it's disrespectful. And a lot of it depends on the maturity of the bride and groom.
Say, for instance, somebody your girl had slept with was actually the person who introduced the two of you... or it's a friend of yours who happened to sleep with the chick before you and her met or something like this. I wouldn't have a problem with those people being invited. But if it's just some random dude she had a couple of one night stands with or some cat who was fukking and chucking her in college or some shyt...