"Get it up," "keep it up"…. These don't feel remotely accurate to me. They suggest that I have something to do with the mysterious hydraulics of my cock.
I've found myself hard on a bus filled with denture-wearing, demented senior citizens, and soft in the mouth of the hottest of sex partners. Sometimes, I think my cock is a barometer of my mental state, my true, subconscious level of desire. Others, I think its turgidity is random, or worse.