No lies here, honest

When my innocent partner was on trial, the prosecution barrister at the end of his closing statement said

"why would she lie?"

I have a better question,

"why wouldn't she lie?"

People lie, men, women, kids, black, white, green with pink spots, it makes no difference! It seems a really stupid argument, but it works, for some unknown reason it seems accepted that a woman wouldn't lie about "that" she would lie about having an affair, about who a child's father is, and hundreds of other things, but not "that" after all there's no reason to is there?