not quite SEXUAL, but at a party once I was...tasting the flavour of this girl's mouth...
...when this guy walks into the area and is like "*ahem*" and I look at him and say "hey...who are you?" and he says "her brother"
luckily for me, I had a good 60lbs on him if he wanted to take a swing or something, so the situation didnt get out of hand...but I was quite embarassed at the time