As fantasy, its fine....as reality, not so much. Endangered species have enough threats without humans circumventing their heat-cycles or rustling the males the females might seek....Pretty damn pretensious of humans to even consider such....since it could be construed as Fence-hopping...of a different and even less-caring sort than usual. I rather doubt anyone has a polar bear to play with, and big cats are like housecats....they often bite and scratch on impulse. They may not mean harm, but putting your naked self where even tame creatures can get to it is a major risk.
And IF you happen to get killed by your object of affection, what happens next is likely euthanasia. Humans have no particular right to keep non-domesticates as pets. Even less do they have a right to monkey about with a threatened or endangered species for the sake of their own vanity. Fantasy should remain a fantasy.