My gut reaction is that there is something else going on here. I don't know what but there is and the two of you need to really be open, honest, and communicate to get down to whatever it is. And maybe it has nothing to do with sex. However, from your post, it sounds like sex can't really be enjoyable for her, just as much it isn't for you. If sex isn't enjoyable for her, why would she want to engage in it with other men? Usually, if someone loves something, they want more of it. If someone doesn't like it, then they either avoid it or try to have it be as unpleasant as possible...which is how it sounds.
When she locks up so she doesn't wet the sheets, is that because of embarrassment or because it hurts? Why did she want to wait to have sex when she was younger but now wants to swap? Did she previously hold beliefs that she now longer holds, which aren't holding her back now? Perhaps the both of you can come to the forums together, research, ask questions, and help you both communicate and understand the root of what may be the problem.
Also, tell her your concerns and maybe she'll open up with her own.