Even saying "most men do..." or "it's in our nature.." is already buying into the defensive explaining, which means you've admitted guilt (to whatever extent) and she's got the moral high ground on you. So--I used to think that is what you say, but now I think it is fatal.
You could flip it on her: "Are you one of those
uptight women?" or "...uncomfortable with sexuality?" Or look at her straight, without apology, and ask "Is there a problem here?" Then turn the ensuing conversation into an examination about her sexual hangups -- !
You can, of course, look at her straight and say "Those are mine and I would thank you to return them where you found them and don't be so obvious about your snooping next time--it's one of those things I really hate in people..."
By all means, never get into an argument about how degrading it is or leads to sex crimes!