I don't 'always' get hard when I bottom, but usually yes, I do. When I top, as long as I'm getting clues from the bottom (erection, or other verbal and non-verbal clues) that he's enjoying it, it's not a turn-off. If the bottom is not putting any effort into sex, isn't getting hard, and isn't making any type of noise, then I could only assume we drank way too much to be enjoying sex, or he's just not having a good time. And if he's not having a good time, neither am I, and it's time to call it quits. If you're really enjoying it, at least let the guy know with some noise, some movement, and maybe a bit of dirty talk.
Have you ever been able to cum while bottoming? It's SO much more intense than when 'not' having your prostate jammed like a punching bag. And when the top can make me cum without me having to touch myself?! Watch it daddy, cause I'm riding you all night! lol