Her

You hang in the void, just as before. You can feel the faintest sensations of Ava underneath you, wrapped around your cock, legs against your hips, face pressing against your neck. But it's as if it's a million miles away, happening to someone else. Instead, the majority of your attention is focused on that choice:

You or her?

As much as Ava seems into it, it feels... reckless to experiment on her without understanding more fully what was happening every time you came. And you're a little excited to see what's going to happen if you pick yourself again. The first time had pretty clearly gifted you your now apparently-irresistible cock; the second, you still weren't sure what had changed. But something must have. Right?

Regardless, you clearly need to pick one or the other. Being stuck in a timeless void for all eternity wouldn't be as fun as having sex with your girlfriend.

Her.

Color flashes back into your surroundings as time resumes. As your hips jam forward, filling yet another condom with your cum. "Oh god!" You cry out, arching your back.

Ava is clenching her way through her own orgasm beneath you and seems to be flexing all her muscles in some sort of writhing pleasure.

As her muscles flex and unflex, you can see them bulking up, putting on ounce after ounce of solid bulk. She'd always been somewhat fit, but now she was gaining some serious msucle mass, and her butt was expanding too. Her abs were hardening, her thighs swelling, her biceps visibly bloating with muscle, and her glutes were being inflated. Her muscles swelled until they were obscene in size, biceps the size of bowling balls, a butt that seemed to hide basketballs within, breasts that I could see from behind her large and flexing back, thighs thick enough to support the enoumous weight of her upper body, and her camel toe seemed bigger as it grasped your dick.

Which was good, because you can feel her becoming slightly shorter, too. In the end her form grew instead of shrank but it lost a few inches in height.

As your orgasm finally settles down, and you see the changes grind to a halt, you marvel at her incredible transformation. Ava really looks like a muscular orc beneath you now -- the top of her head now nestles somewhere around armpit-level -- and you feel like a frail elf captured by her.

You're propping yourself up off the bed with your arms, and because it's a bit strenuous for your new muscles to hold your body weight up like that and somewhat awkward, you carefully relax down on top of Ava, trying to comfort her as you do so.

"Mmmm, you're so good at that, stud," Ava murmurs with a slightly deeper voice into your chest. You give your slowly-softening cock another little flex inside her, and she graples you with her huge muscles appreciatively. "Let's just stay like this forever."

"Sounds good to me," you quip in response.

After another minute or so of you two basking in the post-cotial bliss, you try to pull yourself out of Ava only to find her hug much stronger and tighter than you first realised. When she finally sat up and let you go, the whole bed-frame creaked under her weight.

Ava snuggles into your side. "So, did it happen again?" she asks in a teasing tone.

"Yeah..."

"What did you pick?"

"'her', er, 'you', I guess?"

Ava sits up and looks you over. "Hang on, lemme see if I can figure out what's different..."