has he got a girfriend? maybe he is so used to touching her bum that while drunk his hand just wandered...
similar thing happened to my boyfriend ... he was very drunk and is always putting his hand on my bum... he told me he had accidentally and absent mindedly touched this girls bum while we were at a party (i was there too) they were all standing in a row arms around each other singing, when it happened - and his hand had gone down behind her - and she went a bit weird saying 'im married etc, i dont think of you like that' he apologised profusely, but she wouldnt have it - he hadnt even realised he was doing it.
i totally believe him because he cannot stand this girl.
He has in the past said he thinks shes totally unattractive - she is nasty and bossy and drives them all mad - she thinks my boyfriend and his mates are her 'cubs' and treats them like shes their mother - haha!
but none of the others actually like her either...they only bother with her because of her boyfriend, who is their very old friend
if it was another girl i'd perhaps have been annoyed and maybe had a doubt - but this girl - no way...he said the very idea revolted him.
did he appear to be doing it with intent?
do you think he was actually trying it on, then changed to pretend he was kidding when he saw you werent into it? or was he just being a bit touchy feely and 'handsy' when drunk?
thing is hed have to be very stupid to do that while your fella was in the next room...