AITA for walking in on someone in a public restroom

Hello, this happened to me and my mom a year ago but watching Reddit readings on YouTube have reminded me of this event.

So I (22M) and my mom (46F) went to Publix for some grocery shopping after she had gotten off work, and when we got there we both realized we needed to use the restroom. Our Publix has a women’s restroom to the left, a men’s restroom to the right, and then in between them is a family/disability friendly restroom which is just one large stall. My mom goes to the women’s, and I prefer to go to the single restroom because I am a transgender man and I don’t feel comfortable going to the men’s restroom yet, so I head for that one.

The door was unlocked, so I figured it was unoccupied but of course when I opened it I walked in on a guy mid pee. He shouts “oh shit”, I turn on my heels and close the door behind me, and I quickly walk into the women’s restroom as it is my last resort to take a piss.

My mom and I are the only ones in there, and she asks if something happened because I sounded a bit startled. I say “yeah I just walked in on a guy in the single bathroom, I feel really bad!” And then laugh at myself because of course with my luck that would happen. 30 seconds later, as my mom and I are washing our hands and still talking about it (because when I’m embarrassed I don’t shut up about the thing I’m embarrassed about), the guy comes out of the single restroom and shouts through the women’s bathroom door “KNOCK NEXT TIME YOU FUCK” before storming off and leaving the store, cart full of groceries still in the single restroom and all.

Of course I feel bad because I didn’t mean for him to hear us or to sound like we were making fun of him, but we didn’t really think much of it afterwards until we got home and told the story to my stepdad who said that I was in the wrong for making fun of the situation, because it sounded like we were making fun of him. My mom and I disagree with him, because we were very obviously talking about my luck with social situations, but that’s why I’m here to ask am I the asshole for walking in on the guy and talking about it afterwards.