"I would hit him in his man-parts...." I know they mean the testicles, but this is not specific enough. Is it a kick? A grab? And at what distance would you unleash doom on his 'man-parts'? Even though this is a vulnerable area, timing and the method you use are critical. 

"I would scream..." Making noise is fantastic for so many reasons: it can draw attention to you in a public setting. For an attacker, any extra attention is unwanted attention. A loud, blood-curdling scream can make an attacker do an about-face in a hurry. Making noise can also throw the attacker off,while giving you a shot of adrenaline. Making noise is a big part of self-defense, but what if the assailant comes for you regardless of the noise you are making? What if you are in a situation where noise does not matter? 

"I don't know what I would do, I think I would just let it happen..." I will start by saying that if you have ever been in this situation and you made it out alive, then you did the right thing. You did nothing wrong. I will also say that if you comply with an attacker it does not stop him or her from doing what he or she plans on doing. Being nice or 100% compliant does not make life easier. What if the attacker plans on doing more than just robbing you? It is a scary feeling not knowing what to do and the best plan is to go along with someone's bad intentions. 

Honorable mention: "I have taken a self-defense class before...so I know what to do...I took a class 20 years ago..." Just like any other skill, self defense is perishable. A  doctor, a lawyer, and a judge all need continuing education. Phones need an update periodically. When it comes to combat, fighters train daily. If you cannot train daily, that is okay, but if you have not taken a class in years, then it would be helpful to get a self-defense update. Some of it will be a review, but as the years go by, new and easier methods are discovered.