Yeah, I'm never a stick-in-the-mud. A SITM is that person who never wants to do anything fun. A party pooper.
Actually, though, I do know an INFP that's a big time SITM. I suppose that if the INTP were the sort that took themselves too seriously (a pretentious INTP, par example), or were too socially anxious, then they could very well be SITM. But at that point, it's so much about type-independent traits (like pretentiousness) that it becomes useless to tie it to type at all.