Hold on to your bra straps, Dr Ezekiel (of course he had to have the name of a biblical prophet - it justifies the righteous indignation he feels) Mutua.
Judging from the resplendent mane of hair framing the respective faces of the animals as one, perhaps the more dominant one, the plus alpha among kings of the jungle, mounts the other, we can assume they are both adults. They may have already savagely taken over a pride or two, impregnated a few lionesses here and there, and no doubt killed their fair share of zebra, giraffe and buck.
As adult males, and rather experienced ones at that, we can assume that they are old enough to roar their consent to this form of sexual congress; there might have been a few grunts beforehand to decide who would be the bottom and who would be the top, unless nature took its, er, natural course, and Mufasa took his natural position as lion king - while Simba seems to be enjoying being on the receiving end. Unfortunate choice of names, but can you think of another famous lion's name not related to the Disney family?