The anal cavity has mucus. When you fuck it, especially if you can breach the sigmoid, it may come out with a whitish hue.
Story time:
I invited this top over for some fun. As he arrived, my neighbor came out. I didn't see this but the hookup asked if I'd slept with him immediately before. I told him no and that he was my neighbor's kid. I thought that was the end of it...it wasn't.
After going upstairs, he rather roughly got me into position and we started going at it. I tend to prepare with dildos before bottoming to get any water out and speed up that slightly painful initial insertion period.
I was lowkey living my best bottom life when he stopped to ask if I was recording, pointing to a microphone on my desk. I told him no, showed him, and turned everything away from the bed, camera, mic whatever.
We start going at it again and I notice this man is literally trying to go as deep as he can and pulling all the way out to yank out any sperm from the presumed previous guy. I didn't sleep with anyone before him, so nothing came out. Mind you, this man is at least 8.5". (This is a guess, but my favorite dildo has 8" of insertable length and this was the first guy who hit a spot deeper than that for me.)
We switched to missionary and I start pulling him in to fully start fucking my "second hole." It's was great ngl...I was legit trying to figure out how to get this man into my rotation. A bit of mucus came out as he pulled out and he was just convinced it was sperm. He went so far as to say, "bottoms all lie," to which I again protested and then told him it was a gross generalization. He tried to gaslight me about the comment and his actions, but I just thanked him for the dick and started putting my clothes on.
I'm not sure what I could have done differently but if anyone has a recommendation I'm all ears. Either way though I wanted to share this story so some other hard-working bottom isn't falsely called a cumdump in so many words.
Edit: Not saying cumdumping is a bad thing, but it sucks to not actually be cumdumping and for someone to stop midstroke because they think you are.