I downloaded the "Boolprop TestingCheatsEnabled" guide from this site and read its entirety in Adobe Reader. Nothing in there addressed my problem.

I had a problem with two Sims trying to get married on a residential lot. They got engaged fine, both got memories and engagement rings, etc. When I go to propose marriage, the girl "jumps back" out of the interaction. The guy gets the spinning wedding rings above his head and the marriage memory. She doesn't get anything, and both their icons still show them only as engaged. Their last names are still different. I exited w/o saving and tried again. Same thing kept happening, even under the wedding arch. This has never happened before to any of my Sims. I don't understand what happened because there was another couple on this same lot who got engaged and married without any problem whatsoever. They all live in the same house.

I was told to use Boolprop TestingCheatsEnabled True" and to run debug mode and get an error report. I have no clue exactly how to do this. Can someone walk me through it step by step?

Enter the cheat "boolProp testingCheatsEnabled true" in the cheat window (Ctrl + Shift + C) in Neighborhood Mode. Enter the lot with the engaged sims, and do exactly what you've been doing - i.e. having one sim propose. Most likely, an error message will pop up on your screen. Press "Reset" when it does. Now, de-activate the cheat and exit the game without saving. In the Logs folder located under My Documents, EA Games, The Sims 2, you will find a new .txt file (check the date to find the correct one). This is the error log.

Whoever told you to do this knows how to dissect an error log for problems, hopefully, because I don't. If not, take the information and post it in your thread so others can help you. (You have gone through the FAQ's and tried this with a clean game, right?)