The main idea is that lions does not mean male lions.
It means lions.
Chickens also mate, and indeed cats.
I rarely, if ever, have seen mention of roosters or tomcats mating.
Or bulls, stallions or for that matter, bitches, sows or hens.
So the assumption that the male word is used is incorrect — the general term for the animal species is used, and in many cases that just happens to be the same as for the male member of the species, but certainly not always!
The use of an exclusive single-gender term would be confusing if mating is used to describe the actual producing of offspring — this would, at least in case of most animals, take a male and a female of the species. So using either an exclusive male or an exclusive female designation would be wrong!
When same-sex individuals of a species do engage in a mating ritual (and that is certainly not unheard of) I think it is usually referred to as sex or sexual activity, or indeed mimicking of the mating ritual. I may be completely wrong here, but in my experience, mating is usually (maybe always?) used to refer to the actual reproductive rituals, not any other (social) bonding (sexual or otherwise) between individuals.