Yeah, it is well known that same-gender animals will hump each other during mating seasons when they cannot find a suitable mate because they aren't the dominant male. Its actually a form of sensory and hormone over-load, they become so driven to reproduce they will hump trees, stumps, cars, you name it. They will hump animals that aren't even their own species...so I guess its 'natural' for me to do my cat????
Some animals also hump as a means of asserting dominance, having nothing to do with sexuality at all.