I might suggest asking how, when, and why did you start? Then as a follow up, how has it changed for you you over the years.
These are subjects that are popular discussion points on this board. It may be a good way to get them to open up to you. Also, I might suggest you ask them if shame and or hiding have had an effect on their life. Again it may give you some key insights, but be careful, you may also be venturing into areas where your partner feels vulnerable and nervous about sharing.
Best of luck to both of you!
