Help me understand why he hid this

I am really trying to understand this from the perspective of people who are actually into feederism because this is something I still struggle with.

My partner has a feederism/gainer fetish, and I had no idea. I found out myself about 4 weeks postpartum after we had our baby. I ended up finding out that this had been a part of his sexuality while we were together, including looking at feeder/SSBBW content and interacting with creators, which felt like a betrayal to me.

What I can’t wrap my head around is why he hid it from me.
I’m a plus sized woman myself. I’ve never judged him for being attracted to bigger women. I’ve always tried to be a safe person for him and have always wanted him to feel like he could talk to me about his sexuality and his fantasies.
If he had just told me, “Hey, this is something I’m into,” I honestly think I would have been open to understanding it. I might not have understood it at first, but I would have listened.

Instead, I found out on my own when I was 4 weeks postpartum, which made everything so much harder.
And the weirdest part is that after finding out, I actually started trying to understand it instead of judging him for it. I’ve embraced that side of him. I’ve even started participating in it with him because I realized there are parts of it that I genuinely enjoy too.

I’ve gained about 26 pounds in the last 3 and a half months, and that has been consensual. I like being desired by him in that way, and I actually like some of the dynamic between us.

So I keep wondering why he couldn’t have just been honest with me from the beginning.

Was it embarrassment? Shame? Fear that I wouldn’t understand? Fear that I would think he was only attracted to me because I’m plus sized? Or is this just something people with this fetish tend to keep very private?
I’m also trying to understand how people separate the fetish from their actual relationship. Like, can someone be genuinely in love with and attracted to their partner while also having their partner’s weight or weight gain be a huge part of their sexual attraction?
Because that’s something I’m still trying to figure out.
I’m not mad at him for having the fetish. I’m hurt that I didn’t know about it and that I had to find it myself, especially when I feel like I would’ve been one of the safest people he could have told.

For people who have been in this situation, especially feeders or people who have hidden this part of themselves from a partner, why did you hide it? What were you afraid would happen if they knew? Why would he knowingly violate boundaries in our relationship instead of just sharing this part of himself?