Sounds like your partner may have been profoundly effected by the Evangelical Purity Culture. I grew up with it and fought my way through to the other side. Still know lots of people dealing with it though, even in my 50’s. Half of my friends are like me, but half probably still have sex with the lights out and think oral sex is a sin. This causes them real anguish and that breaks my heart but I can’t do anything except tell my truth.
A couple books I’d suggest that talk about it - Damaged Goods and Pure. The second book is more for women but, truthfully, it could apply to men equally.