A lot of unfounded assumptions being made in this thread. Did she cheat on you? Or, did you actually cheat on her? Is this girl too young for you? Is it possible to get pubic lice from a toilet seat? Etc. Lots of assumptions, very few facts.
One very basic question that hasn't been asked is whether your wife has any pubic hair to begin with? Most women her age nowadays, shave down there. I would assume that's one of the things that attracted you to her? If she doesn't have any pubic hair, then that completely exonerates her, no pubic hair, no pubic lice. Now that doesn't necessarily mean that she didn't cheat on you (it just becomes a totally separate question), just that there's no way that she could've given you pubic lice.
Another fact that hasn't been asked, is whether there are other ways to transmit pubic lice? The answer is yes. Now, the toilet seat method is not one of them, but I wouldn't hold that against her, she was just trying to brainstorm some possible answers on the spot, given the pressure you put her under. But the Mayo Clinic lists possible modes of transmission as shared bedsheets, towels, or clothes. For example, did you stay over at a hotel recently? Did you go to the gym, and was your towel in close contact with other people's towels? Etc. I'll put the link to the Mayo Clinic webpage in the comment below.