"does that mean Patch can't feel it when he gets an erection? :D I couldn't help myself!"

I was wondering that myself. Don't you need physical stimulation for that? That was the reason Annie and Mitchell needed a human girl for Annie to feel sex through in Being Human. As a ghost, she couldn't get Mitchell aroused.

Fitzpatrick never really addresses that, btw. I guess most of the sexing with fallen angels usually happens when they're possessing a Nephilim, but it's still pretty unclear.