I can't say it's such a bad idea but it depends on the context.
Date a girl and test her after two weeks? No way.
Date a girl and test her after three months? No way.
Date a girl, move in with her, get really serious? Maybe.
... want to get engaged or something like that? Maybe more.
Someone told me something important. He told me that people change. That the girl you think you'll love for the rest of your life will someday change. May someday betray you. And he told me to get a prenuptional no matter if she's my "soul-mate" or a Victoria's secret.
Never got so far with any girl but after my fair share of experiences, I can understand the logic behind the statement. You may want to protect yourself from betrayal in some way or another. Getting another guy to test her may validate your ego (if she refuses) but it's very biased.
It's biased because she may understand what's happening or because she may want to fuck another guy she knew for years.
I'm a control freak. I've thought about this many times. But I've also realized that if you have to think about it, simply leave. Less drama, less pain. If your life is about thinking "will she cheat on me?" then you have to ask yourself - do I want to date / be with / love a girl that I'm afraid she'll cheat on me every second? If I don't trust her, why am I next to her?
And here's one more thing - when your instinct says that something is wrong in one way or another, it is. It is almost 100% and the error margin is to low to consider. If your instinct says "that person is simply not right" then it's not. Don't ask me how. Don't ask me why. It simply works. I can say that in 9 out of 10 scenarios, I was right.
But we are so in-love with some idea or concept (like she's perfect for me) that we tend to close our eyes on our instinct when it's only purpose is to keep you happy or at least alive. And why you are with a girl? To be happy. So by shutting down the only mechanism that "predicts" if you are going to be happy or not with a person, you are virtually denying the purpose of a relationship. A claim 22.
Speaking of this ...
My last LTR was pick-ed up using PUA techniques. From date to sex in less than two hours. Meet, spend 15 minutes outside, brought her home, opened a bottle of wine, kissed, kissed some more, talked a bit, candles, satin sheets, fclose. As simple as that.
And there was this guy. Really nice guy who simply wants to be like "the boys". He was in love with her. He warned her not to go at a date with me (I knew her from a friend, talked with her for 40 - 50 minutes by phone). He tried to talk her into breaking up. He was like "this is the girl I always wanted". And still, I've fucked her. So yes, you are kind of right.
Right words, right circumstances and just like I fucked his girl (it wasn't his girl but they were going that way) because I was better, so someone else can do the same thing.
The higher you are on the food chain, the less chance she'll fuck someone else as you can game her better than anyone else. The lower, the bigger
Kind of sucks if you think about it but that's how things are.