the trouble is that is what blokes think.
the very fact that they are invited in is often all they need to make the assumption - disregarding the fact that you have not gone near them all night, consider them a friend, make no move on them, sit on the other side of the room etc etc, but still they think thats it.
sometimes its just because you have had a good night with a group of mates and don't feel like ending it just yet, so you fancy carrying on drinking and having a laugh for a while.
(however, if you have spent the evening with your tongue down each others throats then perhaps its safe to make that assumption)