We've all seen pegging porn where the man is either soft through the fucking or a combination of soft and hard at different times.  Does this mean he is not enjoying himself?  Definitely not!  Let's be honest...butt sex can be an intense feeling at times and can overtake the physical pleasure...even for ladies.  Especially if you are new to pegging or assplay.  The more you do it (and that can include just wearing a plug around a few times a week) the easier it will be for you to adjust to the sensations and focus on your arousal.  It could also be that  you need to communicate better with her what feels good and what doesn't.  Don't forget to kiss and talk dirty to each other to add in more stimulus for your both.  Ladies...if you are flexible, bend down and try sucking his cock while fucking him.  Or reach around and stroke his cock while he is on his hands and knees.  Just remember to be patient and don't get discouraged.  Those hot hands-free orgasms that you see in some pegging porn can happen but are not necessarily the norm without prolonged edging and teasing.