Another common one is "if I don't do it, someone else will". Which completely misses the point that even if true, it doesn't mean that it's ok to do it.

I think it is a utilitarian argument - if its true that someone will inevitably commit the crime then the fact that I commit the crime is victimless or at least had no additional victims than there would have been without my action.

To say that a victimless crime is wrong requires a more deontological approach to morality.