Yep. I still wake up with a semi-boner. Sometimes, they even feel like they're hard enough to pound nails. So, I asked my doctor what the deal is. "Why would I wake up with a good hard-on but they wilt so quickly with sex?"
"It's the strain on your entire body. Your muscles tire and the brain, says, 'Stop doing that, it hurts!' so your erection wilts."
"Well, fuck..." He laughed. "Nope. Ain't gonna happen any more. Masturbate when you can though." So, I do.
Even though that gets tiring as well. My arms give out rapidly. I find myself taking up the old masturbation sleeve. The ribs provide enough stimulation to get me to cum.
Because, oddly enough, cumming not only relieves my aching balls, but delivers enough Seratonin to muscles to relieve the cramping of the Muscular Dystrophy.