Thread title really made me LOL.
Its never really acceptable is it.... to start with its a little embarrassing,
"oh my god, did you just fart" - little smirk, looking at you in shock
"yeah, sorry" - a bit sheepish
Progressing to:
"did you fart again"
"yeah"
"you ****** stink" - and you actually agree with her
until finally you're letting go without a second thought and she's leaving the room in disgust.
So, its never acceptable, but we just do it anyway - you can't hold them in forever after all !