Cats are colony animals. This doesn't mean they're against taxation without representation -- rather, it means they naturally form groups with complex communication between individuals. The most obvious reason your male cat is licking your female one is simply that they like one another.
We often think of cats as aloof and solitary, but they're a bit more complicated than "Man's weird reclusive roommate who poops in a box." In the wild, cats live in much larger groups than they do in most of our homes, crazy cat ladies aside. In zoologist-speak, cats form aggregations, meaning they come together for mutual protection. Natural cat society is also made up of colonies -- extended families of several generations. Cats communicate a lot to each other with their tongues. Cat-on-cat licking is called allogrooming, and it's how they spend a big chunk of their time when they're not sleeping or licking themselves.
Mommy Dearest and Little Rascals
The closest relationships in cat colonies are usually between relatives. Affection between mommies and babies, and between littermates, is lifelong. A large-scale feral cat study conducted by the University of Georgia in 2003 showed a great deal of male-female grooming between adult sons and their moms. Male cats can be surprisingly tender toward their colony's kittens, whether they're their own children, nieces and nephews, or babies of random cat friends. If your boy kitty is licking your girl cat, he may think of her as his mommy if she's older, or as his kitten friend if she's younger.
It practically goes without saying that some boy-on-girl kitty licking action is related to you-know-what. The biggest incidence of male-female grooming in the wild is girl kitties in heat getting licked by boy kitties who want in on the action. If your cats are "friends with benefits," this licking might be one of the, er, benefits. Clues this is the case include having an unaltered female cat, your male kitty doing flehmen -- that crazy, grossed-out looking, open-mouthed face actually means he's really, really interested -- and, um, one thing leading to another, as they say.
Cats, however, don't have rigid sex roles when it comes to licking. The 2003 University of Georgia study showed no relationship between the gender of lickers and lickees outside of cases when a lady cat is having her lady time. Males especially tend to form very close same-sex friendships with lots of allogrooming. Males also form platonic, year-round friendships with female cats. Your male cat licking your female cat signals, more than anything else, that they're very close friends, regardless of whatever reason might be floating around in their little cat brains.
- Journal of Feline Medicine and Surgery: Social Organization in the Cat - A Modern Understanding
- Met Pet: Vomero-Nasal Organ and the Flehmen Response in Cats
- Cat Health: Normal Cat Behavior
- Animal Behavior: Is Cat Licking Normal Behavior?
- Cornell University College of Veterinary Medicine: Cats That Lick Too Much
- ASPCA: Cats Who Suckle and Lick People
- Brand X Pictures/Brand X Pictures/Getty Images