Definitely would be a bad idea for you to pressure your wife to choose between you and her boyfriend. In fact, you realize now that you really do need to pray that HE doesn't ask her to choose, because it's obvious what her choice would be.
Experienced: Should you stay or should you go?
This is based on a true story. My wife really did feel terribly guilty about what she had done. He tried to get her to let him come over again a few more times, but my wife told him no and never slept with him again after those first 2 times.