If this is going to work you need to tell us what prompted this conversation so we can see how plausible it is to pretend you didn't know it was offensive.
As mentioned in another comment, the "first mover advantage" matters when it comes to these matters. The person that initially frames the issue will be given more credence in most cases, because we as a culture have pretty much taken a shit upon the whole notion of "innocent until proven guilty".
That's a legal precept, not a cultural one.
We've culturally never adhered to it as a nation, nor really pretended to.
We've really not done that great a job of it even in the legal realm, at the level of policing. The courts are a better story.