As a professional speechwriter the boundary lines of good taste are pretty clear to me, but if you are writing a best man speech and are a little unsure about whether you have crossed those lines, let me make them crystal clear.
These are your no-go zones. Do any of these and your speech is destined to be a stinker.
In summary keep your content family friendly. Ask yourself this: what would grandma or the bride’s parents think if they heard your speech? If it leaves them feeling sorry for the bride because she appears to be marrying an absolute douche then you have your answer.
If all else fails, remember the golden rule: if in doubt, leave it out.