One time, around 2004, I used a swear word, and somebody said "David, I've never heard you swear before."
A few years later, in late 2007, I was seeing this girl, Jennifer. She said we're doing ok, but we could be doing better if I started swearing more. I responded along the likes of "I'll start f*****g swearing then."
In general I refrain from vulgarity as I can't straddle the divide between being normal and being an uncivilized brute. It's a fine line and a subtle distinction.
So I end up coming off as formal.