My standard response:
“I’m a gentlemen, even if you’re not a lady.”
And for those who don’t acknowledge my holding the door:
“You’re welcome”
I do that too, in my best Dirty Harry voice... :)
When I was in high school in the late ‘60s we rode public transportation. At least half the bus would be students (all boys college prep public high school). Ladies and older folks always got seats. Many times after we offered them a seat they would offer to hold our books on their laps. Nobody thought it was special, because it was normal polite behavior.