There are definitely shower facilities (fee) at the SFO Hairport. Unfortunately, their hours are limited.

I once worked through the night here in Phoenix until just before an international trip (connecting via SFO). I planned to shower before departing to SIN via NRT. Unfortunately, I found the Hairport closed upon arrival. I was desperate for a shower, so I ended up priceline.coming an airport hotel. I checked-in, freshened up, and returned to SFO for the long journey across the Pacific.