For example, I forgot for a moment that a friend had terrible sunburns on his shoulders and his back. I wasn't expecting him to cry out in pain when I jokingly punched him in the shoulder.
I had just forgotten.
Question to you: Have you never forgotten such a thing before?
Something like asking the income of a recently fired family member, only to cause an awkward silence? I did that once too.
It seems like what I did in either case was very rude, but I wasn't really intending to do any harm. I just wasn't thinking. My memory is also very... "selective". I remember all sorts of useless information I read, yet I can't remember what I ate for dinner yesterday night.
I... I seriously can't remember it, now that I think of it. This is... frightening...
Anyway, back to the topic.
Try and focus on the fact that your boyfriend did not do this deliberately. He simply didn't think for a moment. He is still the one responsible for what happened, but there is no "evil" there. He just didn't think things through. So no, what he did isn't "rude", but it is "negligent".
Obviously he picked you up with the intent of being nice / cute. Maybe he didn't think about it. We don't wear dresses so it doesn't always come to mind right away. Just let him know and don't sweat it.
Not all guys are like that. I tend to be very careful when it comes to that kind of stuff, and I don't even have a girlfriend, lol. Shoot, I'm thinking of getting step-bars for my truck, so that when I do have a girlfriend, and when she goes to climb into my truck, she won't have to worry about complications of getting in when she is wearing a dress or a skirt. I know, it sounds kind of dumb, but I tend to think ahead, sometimes a little too much, lol. But anyways, there are guys out there that are careless, but don't mean to be, which happens. I'm sure he didn't do it for THOSE reasons, but just to be cute. Stuff happens...
This happened to me once back in high school when we were on a camp playing a stupid game. The one guy picked me up and BAM BAM BAZINGA I flashed everyone. It was so embarrassing. The more I yelled "put me down!" the more he moved around. But he did apologize afterward saying he's sorry and he didn't even realize that it would happen. So moral to the story, guys forget about these things sometimes. Better to just tell your boyfriend in advance as a joke that there should be no picking up today :)