I don't understand why they would let her do this? Sorry if I missed something, as I'm about to go out and haven't read all of the replies.
My parents had the same rule, and there was no way I could have broken it - I would have been asked to find somewhere else to live, and they would have come knocking on the door asking the boyfriend to leave! Arghhh the embarrassment...
So, yes I think it is disrespectful, but I think your parents are sending mixed messages if they are allowing it to happen.
At this stage (DD is just a baby) my intention is not to let boyfriends stay over unless she was in a committed relationship (engaged/ married), but I'll reappraise nearer the time!