I am going through a similar issue with my female partner. My other lover is male and she feels because he has a penis and can do things sexually for me she can not do without assistance that my sex life with him will always be more fulfilling. I try to reassure her that they both satisfy me, just in different ways, but I feel it is a losing battle with her insecurities. She is genderqueer (accepting that she has aspects of both masculinity and femininity within her) and has always felt lacking in some way based on her absence of male parts. I know this is an issue she must come to terms with on her own before anything I say will make a difference but I'm totally with you on the frustration factor.
Hope your guy learns to trust you and trust himself. *hugs*
My heart overflows and yet still the love pours down... I'm afraid at times I will drown from the sheer amount of it all.