I'm a bottom and there sometimes seems to be no logic to whether my cock stays hard or soft when i'm getting fucked! It's like my dick has a mind of it's own, sometimes i'm rock hard and sometimes totally limp.
However, if I am soft it doesn't mean i'm enjoying it any less. The sex feels amazing (most of the time!) and I will usually get hard and cum at some point. I also quite enjoy the humilation aspect of getting fucked while soft. There's something quite demeaning about my limp, little dick flailing around wildly whilst i'm getting pounded and I also get off on seeing this in porn sometimes. I guess when I am soft, it just feels to me like i'm a totally submissive fucktoy who is there only for the pleasure of the top.