Why was my partner able to enjoy sex at the beginning of our relationship and now they are saying they can’t because they were molested as a kid?

Often times victims of molest can enjoy sex when it is just sex. As they get more connected to their partner, that might change. Think about it. Who has access to kids so that they can molest them? It’s people they trust. Kids who are molested aren’t always afraid of strangers, they are afraid of trusting people and getting into a meaningful relationship where they could be hurt. Adults who molest kids build trust with them so that they can take advantage of the kid without the kid telling. They also build trust with the kid so that the kid feels loyal to them and won’t tell.
Our brains and bodies are marvelous things. We, as humans, have muscle memory that works to protect us. This is one of the reasons why we are at the top of the food chain. Our body will do everything it can to avoid bad things happening to us, especially when we have already experienced something bad. It puts the situation into our memory bank and then it goes out of our conscious memory. However, when we sense that same situation happening again, our body memory / muscle memory kicks in to protect us.
Here is a completely different situation to explain: I don’t have cell phone reception in my house so I always have to use wifi calling. My cell phone coverage doesn’t pick up until I am down the street from my house. So, whenever I walk out of the house, my call will drop and I have to call the person back when I am down the street. This actually happened to me the other day: I was getting ready to go out and was going to call my friend I was going out with to let them know I was on my way. For some reason I felt like I should wait until I got in the car to call them. I couldn’t figure out why. Was I avoiding talking to them? Did I really not want to go out? So, I decided to call them. As I as walking out my door, the call dropped and I remembered THAT is why I was going to wait to call them. I remembered the “rule” without remembering why.
So how does this prove anything? Often times a person who was molested will have different sexual relationships that are consensual. Their body doesn’t see a similarity just yet. Then… they get closer to the person, more connected to the person and MORE VULNERABLE (just like when they were molested… they were vulnerable to someone they thought they could trust). Their body remembers and they don’t enjoy sex but they have no idea why (does the cell phone example come into play here?). This is especially confusing for someone who once enjoyed sex with their partner, and now isn’t enjoying sex with that same partner when nothing seems to have changed.
Depending on your situation when you were molested (for example, depending on your developmental age, who the perpetrator was, if you felt like there were people who knew that didn’t protect you, etc.), you may very well have different triggers, different experiences and different issues. No matter your situation, if you are struggling, get professional help from a therapist who has experience with victims of molest.