Are you planning to send one, or have you just received one?
I am sure there is something about not sending offensive posts. It may/may not fall into this category depending on the opinion of whoever receives it.
If this question is serious, the answer is 'Maybe not written rules, but common sense and normal good manners would put most people off doing this'. I'd advise the recipient of such PMs to report them.
If the Thread is supposed to be funny, I'd prefer it in 'Jokes' etc which I don't have high on my priority list for checking through to find urgent matters to attend to.
Any more sensible answers? Let's have them now, please. This Thread gets closed in ten minutes.