Okay, I really think its offensive if a girl is giving you a blow job and when u cum in her mouth, she spits out in a disgusted way or worst might say "eww", specially if you give her oral sex and you do your best to make her cum!
The first time it happened to me, I got very offended so I came up with this routine and it has worked for me on every girl who's spat out and my female friends have agreed it'd work most of the time!
I say I came up with it, but there could many of uz out there who use the same thing! :p
Here is the actual msn conversation I had a few days after I met her for the 2nd time and got a blow job and she spat out. For obvious reasons I had to remove both our email addresses and user names out of this but all the content is untouched.


...
Me: oh one thing
Me: :p
Me: do u like to get licked?
Her: down there?
Her: :$
Me: yeah
Her: obviously
Her: lol
Her: shutup
Me: ya
Her: any1 who says they dont is lyin
Me: lmao
Me: i love doing it
Me: anywayyssss
Me: u see
Me: remember when u spat my cum out?
Me: and said gorss?
Her: coz i dont like cum in my mouth atttt alllllllllll
Her: did i offend u?
Her: im sorry
Her: its not personal lol
Me: well was the first time some1 spits out
Me: but
Me: thats a good enuff reason for a guy not to lick ya
Her: lol
Me: cuz he has to constantly swallow ur wet and cum
Me: and how u feel when he goes down on ya and licks ya and spits out and says gross?
Her: i just dont like how its slimy and it got stuk in my throat lol
....

Anyways the conversation got drifted to other stuff after that which is irrelevant to the topic, she was an 8, from a middle eastern back ground and I was the 3rd guy she had slept with (well she was in a long distance relationship with her boyfriend who lived in Norway or something like that and she never cheated on him before me), and how I got to get her number and stuff was one of the BEST game plays I've done but again not relevant!


Next meeting up was the sat of the same week and she swallowed this time and the next time after that!

Since then I have tried this conversation with a few of the girls I slept with (saying that makes them a "hypocrite" and so on... and they all ended up doing so.)
However as one of my female friends noted, it won't work if the girl you sleep with does not enjoy to be licked, or you are not very good at licking her, as she might say "well don't lick me then"!

Try it out and post ur result!