A little standoff-ish at first, but nothing a few questions can't fix.

When Princess Luna finally landed on the balcony, Isaac could feel the vibrations through the floor. The moonlight was more dim than the sunset, but there was no mistaking the blue-furred Princess's massive booty jiggling firm and proud and wide behind her, just as full and voluminous as Celestia's tits. Her thighs were massive as a result, too, tapering down to her relatively dainty hooved feet. Above she had a surprisingly thin waist and reasonably large blue breasts twice the size of her head covering her torso, framed by a normal pair of shoulders for her size, and then a beautifully feminine equine face framed by a mane of glittering starry constellations, her mouth... frowning at him. Uh-oh.

She took a few steps into Celestia's bedroom, each step making her ass behind her quake and the floor rumble every so slightly. She stood a little shorter than Celestia did, maybe only about nine feet tall, but that still meant she had almost three full feet of height above Isaac. When she stopped next to the bed, her ass still kept jiggling back and forth, visible even from the front due to her hips stretching far wider than her shoulders. If Celestia was an ancient people's exaggerated idea of a wet-nurse, Luna was an ancient people's exaggerated idea of breeding hips. Isaac was against anything to do with childrearing for many reasons, but his natural male instinct to mate was activated by her powerfully oversized form.

"Is this meant to be thy latest boy-toy, sister?" Luna scoffed, folding her arms beneath her bare breasts. At first glance it looked like she was wearing nothing but her royal regalia, but in fact she had a gauzy silk cloth draped in front of her fat pussy, and a sparkling but understated jewel corset around her narrow waist.

Celestia returned behind Luna and hugged her from behind, draping her arms around Luna's shoulders. This had the effect of planting Celestia's gigantic white tits atop Luna's gigantic blue ass, a sight that made Isaac's loins tense up preemptively. "The best one yet, I daresay! Why don't you take him for a ride~?"

But even with that ringing endorsement, Luna didn't look convinced. "I know your tastes well, Celestia. But not everypony likes them meek and humble. This one doesn't seem to have the strength of heart to truly take what he wants. He watches me in slack-jawed awe, like he doesn't deserve to be in my presence - hardly the attitude of a true Master."

Isaac blushed and rubbed the back of his head, totally caught. "You can't really blame me for being excited on my first day, right? Don't you think your Master should be delighted to see a new sexy **** for the first time?"

That seemed to cool Luna off a bit, her eyes widening in surprise before glancing at her sister nuzzling into her shoulder. "His very first day, truly? He's charmed his way into your bed and given you such confidence in a single day?"

"Not even a full day - just one afternoon," Celestia clarified with a confident grin. She reached around and gave Luna's tits a sensual rub, cupping them from beneath and then gliding over them in broad circles, making her younger sister moan. "Even Rockhoof took almost a week of working up the nerve before he took up the challenge. Isaac here?" She leaned up to Luna's equine ear atop her head. "He fucked all my holes in mere hours. He's no ordinary male - he just wants to take and fuck."

Luna shivered with the lustful implications of that statement, but with a dismissive wave shooed Celestia off her shoulder so she could stand alone again, hands on her incredibly wide hips. "Very well, Isaac. I will consider you endorsed. But I hope Celestia has let you know that the Harem Master title comes with very high standards. I will need to test you before I can have full confidence." Her horn lit up, and Isaac found himself magically levitated off the bed, weightless and helpless in her telekinetic hold. "Let us see how you do without Celestia's help and encouragement." She then turned around - presenting her body-smothering ass in all its glory - and pulled Isaac out onto the balcony with her.

"Remember what I told you," Celestia half-whispered as Isaac floated by, smiling encouragingly. "Be honest with yourself. Take what you want with confidence."

Luna unfurled her navy-blue wings and took off, making Isaac panic a little as he was pulled along high into the air above Canterlot castle! Thankfully, the trip was short, from one tower balcony to another, but it was just long enough for Isaac to remember he had a fear of heights! It was also just long enough to remember that Canterlot was atop a mountain, meaning he could see the plains of Equestria under the dark of night for just a second - an incredible view.

Then Luna landed again, this time in front of her own bedchambers. She released Isaac from her magic a couple of feet off the ground, which made him scramble and reach out for something to steady himself... which ended up planting his hands deep in Luna's ass-flesh.

And what a fucking glorious ass it was. His hands sunk in deep, but he could feel a solid core of pure gluteal power beneath the layers of fat. This was a mini-giantess that could probably crush entire rows of melons between her cheeks or her thighs - a thought that both intimidated Isaac and deeply aroused him on a fundamental level. He found himself rock-hard and drawn deeper into her ass by magnetism, lurching forward and burying his face and upper body against her huge, wobbling cheeks, his cock bobbing just below.

Luna let out a slight cry of surprise before looking back over her shoulder. "Forward, aren't you? Art thou truly the kind of pervert that enjoys a posterior as freakish and bubbly as mine?"

Isaac pulled back just enough to look up at her while still hugging as much booty against him as he could. He realized that there was an effect he placed on Celestia that he'd neglected to put on Luna. "Aren't you just as proud of your ass as Celestia is of her tits, if not moreso?"

The Princess of the Moon's posture changed quite significantly. Instead of being tolerant of him hugging her ass, she was actually thrusting her booty back against him, wiggling her wobbly cheeks against his face, and brushing her star-filled tail atop his head. In fact, her ass felt a lot more firm and toned and smooth, like she'd put a lot more effort into it over the years! "Celestia can keep her milky funbags," Luna crooned. "Everyone knows the quickest way to a male's heart is through the biggest, juiciest, most fuckable ass~!"

"Well, fair warning, I'm a boob-man at heart," Isaac replied with a playful chuckle. Which had the not-unforeseen effect of making Luna push her ass further against him until he was backed up against the balcony's railing, her ass smothering his whole upper body like it was a lost TV remote between couch cushions.

"Then I guess we shall have to convert you the best we can~," Luna teased, then opened the doors to her private bedchambers and walked in, releasing Isaac and leaving him slumped against the railing with a dizzy, heated look on his face. She looked back at him over her shoulder with a sultry, smoldering look in her eye. "Come in, would-be 'Master,' unless you think you cannot handle the power of the Moon~?"

Isaac had a moment of reflection, mostly along the lines of, Oh god, I've created a monster. Then he remembered that being surrounded by impossibly sexy, amorous women was the entire point. He'd never jumped up to his feet so fast.