"A key that opens many locks is a master key, yet a lock that is opened by many keys is a shitty lock." This analogy or variations thereof are assumed by sexists and idiots alike to be an elegant way to defend why a double standard exists in (heteronormative) sexual practices. To be sure, it is elegant. It is also spectacularly stupid.
That's because it applies circular logic. It simply restates the double standard that female sexuality is somehow a room, presumably full of valuables or secrets, whose contents are finite and therefore more interesting the fewer keys that can open it. Conversely, the key is as the key does: no content, no hidden layers, nothing of note to be treasured or cherished, only in as far as it is capable of opening rooms. It's like the world's dullest dick joke made to restate the double standard.
There is no inherent value to sexuality other than what cultures construct it to be. The double standard causes grief to both men and women, subjects women to a myriad of hoops to jump through (though they can still be a "slut" even if they've had sex with just one guy if other people think the circumstances were wrong - so you can't win as a woman, really) and depresses men into becoming shallow idiots whose main source of pride is how much women they can bend to their will.
If anything, the key/lock analogy presents a piece of patriarchal yarn in its starkest and bleakest way, and shows us how stupid it really is.
