My fiance did something weird this evening and I feel a little weirded out by it

My fiancé (35 M) and I (28 F) go to the gym weekly and park in the same parking area. For about five minutes, we’ll have a conversation sometimes longer but lately it’s been pretty fast paced because now that we have started the work and school semester. It’s been kind of crazy. He’s a teacher and I’m a student for college. anyways I wanted to mention that today we held each other and hugged each other and as he was walking me to my car door I turned around to joke with him a little bit and from the distance I can hear “kiss! Kiss! Kiss!” and this black SUV I’m assuming was a Dodge passes by a little fast but it’s what I assume were two young kids screaming this from the passenger side of the car. It sounded like kids I can’t imagine it being adults but I would say maybe 10- 13 I wasn’t really sure it just the voices were super young- maybe they were teens? The thing is as we’re walking to my side of the car, which is closest to the street that the SUV is speeding by my fiancé raises his arm up and throws the finger. And I knew that he did this, and he confirmed this by telling me that that was what he did, but I couldn’t help but look down in shame when he did that because I would never do that. He told me that he didn’t like people bearing into our business, and that’s why he doesn’t really like showing PDA. He started to be a little paranoid and said that we should probably park in a different area next time we go to the gym. The street that this SUV was passing by is from a neighborhood. I assume that that family lives close by, but I couldn’t help but feel a little upset at the thought that we would have to park Somewhere else because my fiancé chose to do something ridiculous. Was this a valid reaction? I don’t know how I felt about this. All I could say is that I hope that he has an ounce of shame because I don’t think the reaction was appropriate. Can someone help me try to understand his thought process even though he explained it? He says he thinks people are weird and shouldn’t be yelling out from a car or giving an audience.