If you are digging the rope -- look up a dungeon and take a rope class. Learn to do it safely. It can be fun. And remember... like with anything else... "my rope style is not your rope style."
To me? You did fine. You asked permission, you do realize your joined THEIR already in progress scene.
You don't know what they agreed to as the boundaries for that scene, and if the D/s overtones was for that scene only or a 24/7 arrangement or what.
I see that you prefer to know that stuff from the person's mouth directly, but the scene was already in progress. Maybe they didn't want to stop the mental flow. Or maybe he knows something you don't -- like kissing on the lips when she's in subspace wigs her out?
Next time if you want to honor your own preferences... you negotiate that up front before the scene starts with the players if you are going to participate. But for stumbling in on someone else's thing being played out in a "public space" you asked permission and they could have said yes/no to you.
Dude's response? Well, it seemed to push his buttons some how.
You could apologize for pushing his buttons inadvertently.
You could ask him to apologize to YOU for attacking your character and telling you that you are horrible rather than giving you feedback on your behavior. (That was not inadvertent.)
You could ask him if he's willing to clarify how your behavior was wrong.
Could remind him you ARE allowed to have your own preferences -- just like he gets to have his. You are allowed to discover you HAVE some preferences in this new world in these new situations. Could ask how you having preferences of your own is preventing him from having his preferences?
Or you could let it go. (Or could do something else I cannot think of right now.)
Maybe Dude's preferences/experiences frown on "topping from the bottom" once a scene is in progress. And the sub in this case speaking up falls in that zone to him? So your stating your preferences is pushing that button?
On the flip side... some people LIKE topping from the bottom. *shrug*
Or maybe he felt judged or triggered by HOW you expressed your preference? Not so much that you have one, but HOW you shared it with him? Only he knows. :/