More interesting question: have you ever watched a porn...with someone else?
With a GF I find it's an interesting way to fan the spark, so to speak, though it's not a good idea for the man to suggest it unless he is confident she'll be amenable to the idea. Also, the TV will be ignored after 5 minutes anyway. But it's a good way to get her excited by being "naughty," no matter how trite that may seem.
And what's with watching porn with other (presumably heterosexual) dudes? I was drinking with a buddy one night and watching Family Guy on his Tivo while he tried to fix his computer (I didn't yet know that he was doing meth) when his new "friend" (read: dealer) came over with a $100-dollar, 4-hour, multidisk porno and put it on without much preamble. He showed us the "easter eggs," the special features, the soundtrack CD and a scene with an amply-augmented blonde singing "Like a Virgin" while spanking the microphone guy before putting on the actual spank material and turning on the DIRECTOR COMMENTARY. The whole time my friend just snickered while he tinkered away at his computer. I was aghast at this total stranger watching bondage lesbians sodomize each other with fist-sized toys and talking casually about the numerous actors, actresses and directors in the porn industry who's work he so admired. At one point he spoke of some starlet and how he'd been (direct quote) "following her career for some time." At this point, I couldn't resist (and I wanted the movie turned off immediately). I asked if her husband had told him to stop calling yet. He looked genuinely hurt. And the spank film was turned off.
This happened another time, but as that story involves projectile vomiting I'd imagine y'all don't want to hear it.