Kaworu and Shinji were passionately touching each other. at this point They felt like nothing could stop them in their act as their underwear gets slightly wet from the built up pre-cum.

"Shinji.. Shinji I want to do it with my mouth"
Shinji's cheeks turned heavily red and he felt like he could faint any time, but he knows that he can't turn back now and he whispers into Kaworu's ear "okay, please"

Shinji lays on his back and relaxes and Kaworu passionately moves down the lower part of Shinji's body. He then slowly starts pulling down Shinji's tight underwear.

The boys are ready to start doing their thing..
Shinji adjusts to better position by bending his legs..
Kaworu slowly places his head in Shinji's crotch and then suddenly...
"KAWORU"
"Huh?! what is it Shinji?"
"I-I-I.."
"You what?!"
"I-I gotta"
"YOU GOTTA WHAT, JUST SPIT IT OUT"
but before Shinji can phrase his awkward sentence, he aggresively farts into Kaworu's face.. The fart is so intense that strands of Kaworu's hair could be seen bending backwards.. This intense sound pierceed through the awkward silence in the room..

Kaworu is absolutely shocked and has no idea how to react.. After about 10 seconds of more awkward silence, Kaworu bursts out shouting:
"HOW RUDE, YOU HAVE SOME NERV!"