"Mmmm. Just relax." She broke off the kiss and knelt down in front of you, pulling down your pants and reaching into your tented boxers. You ran your hand through her hair, playing with it as she grinned up at you, stroking slowly and giving you an occasional teasing kiss through your stretched boxers.
"Ready for your surprise?" You nodded eagerly and pulled her in closer as she slid down your boxers and grabbed your dick, running a long lick up the underside and taking the head into her mouth. A hand pulled down the back of your boxers and you felt a lubed up rubber hose slide up your butt. Your girlfriend pulled away from your erection.
"Surprise!" We both yelled together. Your girlfriend giggled.
"I can't believe you didn't notice me sneaking up on you." I said, slapping your hand away from the hose. "And you thought you could get away with cheating without us finding out, too." Gas was rushing into you fast, bloating your stomach up in front of you.
"Well, this is really awkward. I guess I just didn't expect you two to ever meet and figure it out." You blushed, more out of sexual stimulation as you blew up than embarrassment. Your hips and ass had blown up to match your swollen belly. You had the gravid bulge of a pregnant woman, and you were still growing larger as helium gurgled through you.
"Wow. We work at the same job, and you didn't think we'd ever figure it out? Are you serious?" Your girlfriend looked incredulous. "Yeah, let's blow him. He deserves it."
She leaned back and took a deep breath, her chest and belly both filling up, before wrapping her mouth around your dick and blowing hard. Just as she started to blow, I cranked the flow rate on the tank all the way up to full.
Your body creaked dangerously and the pressure inside you skyrocketed as you quickly doubled in size, widening out as your pneumatic gut and overinflated bottom tightened. You felt like a human bomb, full of absolutely overwhelming pressure, about to explode right there. Your body let out another long, loud creak.
Suddenly, the pressure abated as your girlfriend stopped blowing and sat back, panting for breath. I turned the tank back to half pressure.
You had blown up really big really fast, but we hadn't managed to make you go boom. Your shirt was stretched tight over your inflating body, and your boxers and pants had slid down off your ankles and onto the ground, leaving you completely exposed from the waist down. I wrapped a cord around your ankles and tied you down as a precautionary measure.
"Whew. I think we might need more pressure." Your girlfriend said, still out of breath as she walked over and pulled another helium tank out from behind a bush.
"Oh, you brought one too?"
"Of course!" She chirped happily, obviously pleased with her foresight.
"You're so smart. This-"
"Hey! Why don't you two go get a room?" You interrupted from over your bloated body, still tightening as more and more helium flowed into you. You lifted off the ground and bobbed against the cord tethering you to the earth.
"That was the plan, actually, as soon as we're done with you." Your girlfriend snapped back. She slid the hose on the other helium tank into your belly button. Your shirt burst into confetti, leaving you naked.
"Okay, I'll take top, you take bottom." She nodded and got back down on her knees, taking your overinflated member in one hand, tank valve in another as I stood over her and faced you, hand on the valve of the other tank. "Ready? 1... 2... 3!"
We both inhaled deeply, bellies filling with air, and twisted our tanks to full blast and blew as hard as we could as I kissed you on the lips and your girlfriend blew into your dick.
You swelled rapidly, creaking, the pressure inside you so intense that you felt like you were in the process of exploding as you widened and grew rounder and tighter. You creaked loudly and your inflation slowed as you reached an apex of pressure, and you watched my eyes widen with surprise as me and your girlfriend were overcome by the pressure and started to bloat with helium.
We held on, fighting against the pressure as the three of us inflated, pushing hard to make you explode as we swelled and our clothes tore to shreds. With two people and two tanks filling you, it didn't take long. You groaned, creaking loudly like some huge, loud object that explodes, crested over your limit, and rapidly bloated out several feet in every direction.
Stunned by the blast and not tethered to the ground, your girlfriend and I flew upwards. By the time we had shaken off the shock of your sudden detonation, we were 40 feet in the air. Our naked bodies rapidly floated higher and higher, swelling bigger and bigger as the atmospheric pressure around us dropped. Your girlfriend creaked first, loudly.
"Oh shit." I gulped.
"Might as well make the most of this, right?" She reached over and pulled me in closer as we shot into the sky together.