Turn-about is fair play if you aren't going to be straight-up about things.
Another analogy would be like finding a way to make sure that your resume isn't one of the ones that is immediately thrown out by some HR person before being forwarded to the hiring manager. The hiring manager still gets to evaluate you based on your skills, but you just made sure that he/she even looked at your skills (vs being cut from the list for some arbitrary reason).
On the other hand, a girl having her best friend hit on you to see if you'll cheat demonstrates a fundamental lack of trust in the relationship. Regardless of the outcome, she's already demonstrated that she doesn't feel that she can trust you. This is a far deeper cut than convincing all of the extremely 'hunky' guys at a bar to leave (for some reason or another; maybe spread a rumor that the bar next door is giving away power tools or hair gel or something) so that you even have a chance with some of the attractive women there.
For some reason, it's just the 'pretending to be someone that you are not on the internet' that makes people throw up the 'creepy' flag. If I was to pretend to be someone that I'm not in real life, people would not immediately think that I was creepy. Hell, society seems to celebrate guys that lie about how successful they are in order to impress women.
However, I have a feeling that the intersection of the sets "Guys who meet girls through large-scale subterfuge" and "guys who get deceptively set up by their fiancees" is not large enough to be worth addressing in a comment like that.
If that happened to you, it's one woman who did it, not "they".
I guess I took the "when" as a colloquial "if" - if your reading was more literal then I suppose I can see why you're upset.
Than I realized what you really meant.
Now that I think about it, maybe relationships aren't the goal of this kind of behavior in the first place, so maybe this whole thread is a really bad idea for HN.