Despite how powerful and masculine you've become, you're missing one big part. The one betwen your legs, to be blunt. Though you're well aware that most apes are far smaller than humans, both proportionally and in inches, you still don't think your still human-sized shaft and balls are quite done changing. You part the fur that covers them from view, absentmindedly munching on a banana in the meantime.
As if on cue, the changes start. It feels like someone punches you in the gut from the inside, startling you into letting out a suprised "ook". Your hips jerk forward, legs moving out to the side as you recover your balance. A bulging feeling starts growing within your pelvis, like someone's kneading or massaging the flesh between your bladder and shaft. You try to remember your anatomy classes when it finally hits you. Your prostate is growing, getting bigger so it can properly provide for your now enormous body. It grows to twice its old size, a teasing tenderness and sensitivity making you clench the muscles of groin as if someone were fondling you teasingly. Mercifully the sensation soon stops, the new mass settling down as the changes move on.
Your hips jerk forward again, this time with a surge of pleasure and an involuntary tensing of your abs. It's like you're trying to come but can't, and the frustration and pent-up energy descends into your balls, which churn and groan with need. You breathe unevenly, incredibly turned on but feeling helpless to do anything about it. Your mind is in a haze, too distracted and turned on to manage much at all, your hips bucking over and over without any release. The churning grows within your balls, which bulge and turn heavier with each buck of your hips, your scrotum stretching and tightening with the increase in weight. Your balls are the size of baseballs now, far bigger than a person your size would have, and far, far larger than a gorilla would. Whatever's changing you, it's taking the best part of both species and turning it up to eleven. Your balls finally finish growing, filled to bursting with seed as if you'd gone lifetimes without coming.
You're grunting and huffing now as your hips buck, desperate for release. The next rush of pleasure latches onto your shaft, instantly making you harder than you've ever been in your life. The uncut tip of your cock peeks out from under your fur, the tip peeking out from inside the foreskin, the deep purple head already glistening with precum and begging for release. You buck your hips again, and the motion seems to travel down your dick, driving the tip outward, adding almost an inch to its length, and widening the entire shaft as it goes. You buck again, harder this time, adding nearly two inches this time, the shaft half again as thick, still a dark black like the rest of your skin. The surges of pleasure keep coming, driving your arousal and need to greater heights and making your frantic for release. Another two inches, thicker still. Your cock is enormous now, ten inches long, and as thick as your thumbs, over two inches across. Veins dance under the surface, pulsing in time with your sheer, primal need. The haze and lust in your mind thickens, and if you don't come soon, it feels like you could pass out from the urgent helpless need you're feeling.
Finally, you're given mercy. A final surge of pleasure pulses through you, three times as powerfull, and whatever's stopping you finally lets you. The orgasm surges through you, doubling you over as every muscle in your body tenses, your cock pulsing and wriggling as it unleashes a torrent of thick seed, splattering the mirror before you as you grunt and roar. You keep coming, down on your feet and knuckles, groaning deep and loud with an orgasm that's broader, deeper, and more profound than you could have ever imagined. Finally, the gush of seed slows, the last bits dribbling from your quickly softening member. You're already getting sleepy, but you force yourself to grab the wipes and scrub brush from the bathroom cabinet, cleaning up the mess you've made, and then your own sticky fur, before finally letting yourself wander off to bed.
You pull the covers over your new primal form, and let sleep claim you.