I know a lot of people will get on me for this, but gosh...I HATE it when a girl farts on you...ESPECIALLY DURING A 69 position. Come on man...it stinks. My penis literally becomes flaccid quickly after she farts. It's that smell that just can totally block your cock. I'm sorry, it's just me.
No it would not be a turn off for me, I mean it maybe a bit unpleasant but I would not be turned off because I know it would be an accident. It is not like she would have been trying to work the gas down and push it out in my face. Therefore I would expect her to apologise but I would tell her not to worry and I would carry on with sex with the girl.