ANSWER:
Firstly, congratulations on having the courage and determination to work through this. It is possible to work through infidelity to have a stronger relationship than ever before.
I know it won't be easy. There are going to be many layers of this to work through for both of you and this will be a journey that takes some time.
When you say your partner equates sex with pain, I'm assuming you mean emotional pain from the fallout of the affair. If it's actual physical pain, my answer would be somewhat different.
I'm guessing that your partner is struggling to maintain his erection because his sexual desire/turn on reminds him of the ways he hurt you.
Try changing your focus to sex being about pleasure and intimacy between you.
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Don't worry about him not having an erection for now. Focus instead on rebuilding your connection and your physical intimacy. Even without him having an erection, it's possible to enjoy physical and sexual connection that brings you closer together. Do things that are possible and that you both enjoy. Focus on your pleasure right now too.
As your partner re-associates sex with closeness between you, it may help change the connotations in his mind.
Have him explore whether there are certain aspects of sex that he finds more strongly associated with pain right now. It might be helpful to explore some of his current underlying sexual beliefs with a therapist by himself, so he can voice more of what's happening for him without worrying about hurting you.
Erectile dysfunction medications may be helpful as a short-term support. The short-term use of medications, while a person works on the underlying psychological/emotional causes of erection difficulties can help build confidence and positive associations with sex.
Relationship Rehab is news.com.au's weekly column solving all your romantic problems, no holds barred, with resident sexologist Isiah McKimmie.