And his nature DOES take it's course!
Jack growls out in a deep baritone, "Oh man, you know, I was never into guys before, but I think I've changed my mind about that."
He then pushes his long thick member up your ass. You let out a howl of pain but you're too horny to even think about stopping, instead, you do your best to just grit your teeth and bear it (no pun intended).
Jack moans, chuffs and growls. He doesn't seem to care that he's not human anymore and you are extremely glad he isn't. The feel of his soft belly and chest fur on your back is heaven, his warm musky scent makes your head spin with lust and the feel of his cock in you has slowly gone from burning pain to incredibly pleasant in the last ten minutes or so.
You feel his sperm in you belly, gurgling about and it is soon joined by more thick hot semen from Jack's huge member. Jack roars in pleasure, but he doesn't go soft, indeed he starts pounding harder to bring himself to a second orgasm.
He grunts and between those he tells you he was a bit mad at you for tricking him into the costume, but now, now he's glad you did because he loves the feel of his massive, horny, hairy body.
And speaking of horny and hairy, you begin to notice that your arms are starting to grow fur in much the same color as Jack's fur. You feel your muscles inflating and thick padding growing over that muscular bulk.
Jack shoots another five loads into you and you shoot directly after, covering your crotch in heavy cock cream.
"Fuck! I can't believe I'm still hard!" Jack rumbles and continues his rutting.
Slowly as he fucks you, his body changes; he's losing the form of full grizzly. His height decreases to something over six and a half feet tall and at least four foot wide but still covered in a full pelt. His hands become hands again, though they are padded like paws and have a thumb. Jack speeds up yet again and shoots into groaning and growling in intense orgasmic pleasure.
You come in unison with him, not realizing that your cock has grown, your balls have become the size of goose eggs, the same size as Jack's and fur now covers your husky taller frame. Jack stops shrinking but his cock I still rigid and he is still horny.
"Oh God, I can't believe I'm going for round four!" He says as he begins thrusting again.
"Oh fuck Jack!" You growl in a now deep bass voice, "Don't stop!"
You are as horny as the bear now, and almost his equal in form.
Jack pulls out, still long and hard, flips you over to face him and then reinserts himself into you. You now lie face to face. He pushes his long, thick, ursine tongue down your throat and you kiss hungrily as he fucks you. Your cock rubs against his belly as you fuck. Your change completes as he shoots for a forth time into your stretched out asshole. You are now his match and shoot just as long and hard as he does, splattering the underside of his muzzle with your sperm, coating his furry chest in hot white seed.
Finally, after the fire of orgasm fades, Jack sighs and lies on you. The beast you both have become is finally satisfied. The pair of you ache from your transformation and exertion but neither of you care, your are both spent and need rest. Your anthropomorphic ursine forms; bulkier, stronger, brawnier need sleep despite their awe-inspiring power.
You both settle into deep sleep, Jack still inside of you. As you sleep, you revert further to your final human forms.
Jack is a lot bigger than he was and now, it isn't all fat. You are too, and now it isn't all muscle. All in all, you're both far more hairy than you started and both of you have full thick beards and though you may think the bear is gone, somewhere in the back of your sleeping mind, the bear within speaks to you both of his comfort residing in your body.