Brace yourself for a guilt trip: July 24 — that's Sunday — is Parents' Day. We're willing to bet you haven't bought a card. Why does Hallmark make us jump through these hoops?While everyone else is toasting mom and dad, we thought we'd highlight a few famous folks who, for whatever reason, never reproduced. Some were (and are) stepparents, some raised animals, and some, as Jennifer Aniston recently put it , don't accept the outdated notion that they are "somehow incomplete, unsuccessful, or unhappy if they’re not married with children." More power to 'em.Here, 15 child-free celebrities sound off on why parenting never panned out for them. In the cases of Aniston and longtime fatherhood-denier George Clooney, it's a matter of never say never, but there's no judgment either way. There's no shame in putting career above kids or simply not wanting to add to the planet's population.Besides: Isn't Betty White everyone's grandma by now?