The new proposal by Supervisors David Chiu and Sean Elsbernd to make it easier to take baby strollers on Muni buses sure prompted a lot of whining — and not from the actual babies.

Comments on our story about it on SFGate.com included “If urban parents can’t handle the urban, perhaps they should flee.” And, “It’s about choices, people, so if you have a ton of kids, Stockton is perfect for you.”

With that kind of attitude, perhaps it’s no wonder San Francisco has the smallest percentage of residents younger than 18 of any major city in the country, according to 2010 census figures.

I decided to test whether Muni passengers are any more polite to pregnant women than they are to parents of little kids. The guinea pig? My sister, Beth Hawkins, who rents an apartment in Pacific Heights and teaches fifth grade at a public school in Marin. She graciously gave up several hours of her spring break to spend Monday morning crisscrossing the city on crowded buses with me.

Yes, her birthday present will have to be extra special this year.

She’s due with her first child in a month, and she’s also one of those annoying pregnant women who remains skinny everywhere but her belly. And I forced her to wear a belly-hugging shirt for our experiment so that there would be not doubt that she’s pregnant.

We started on the 38-Geary headed downtown and were pleasantly surprised when a young man listening to music and wearing a Madonna T-shirt offered her his seat. That would be the one and only time all morning long.

A woman with her purse sitting on an empty seat next to her looked right at Beth and did nothing. Several other people looked straight at her belly and then straight at the floor.

When the driver of the 45 Union-Stockton headed toward the Marina seemed to think he was actually participating in the Indy 500, a woman put her hands up so Beth, who lost her footing, wouldn’t fall on her. And then the woman remained in her seat.

On the 8-Bayshore headed back downtown, two hippy-dippy women had a long discussion about rainwater collection, volcanoes and the glory of Mother Earth. That wonder certainly didn’t apply to actual mothers-to-be, as they pretended there wasn’t one standing right above them.

Final tally? One well-mannered rider out of several dozen who had seats. I asked Beth to describe the attitude of her fellow Muni riders. “Mostly indifferent to mildly annoyed,” she said.

Sounds familiar to Angelie Agarwal, co-founder of the Muni Manners website, which offers 50 etiquette rules for Muni riders. (It seems obvious that people shouldn’t belch loudly, fight or carry open and sopping wet umbrellas on board, but apparently not everybody’s received the memo.)

Agarwal, the mother of a six-month-old who lives in Glen Park, rode Muni to work at least once a week when she was pregnant — and was never offered a seat. She noticed BART riders were much more friendly and accommodating, readily giving up their seats and asking her how far along she was.

“People on Muni could care less. I was an inconvenience to them and a conversation topic on BART,” she recalled.

Officially, Muni policy doesn’t address pregnant women. Spokesman Paul Rose said, “The rules state that passengers should make seats available to seniors, persons with disabilities and other passengers when needed.” He said Muni is looking at a variety of ways to make its buses more family-friendly and accommodating.

Mayor Ed Lee, through his spokeswoman, said he supports Chiu and Elsbernd’s baby stroller legislation and wants Muni to become more family-friendly in general.

Rose said Muni plans to get feedback on proposals that could allow parents to carry their strollers up the stairs or to request a ramp and leave their kids strapped in, or gasp, even have a designated area for strollers.

P.S. We struck up a lively e-mail exchange with Paul Tominac, a Muni bus driver for seven years, who wrote in response to the baby stroller legislation story.

He went on to lament the lack of common sense and common courtesy among Muni riders — like the two women who boarded with folding carts full of groceries and took the two front seats opposite each other, leaving six inches in the aisle for everybody else to board.

He’s seen people try to board crowded buses with five huge garbage bags filled with recycling, a full metal security door and frame and even a woman who was using Muni to move to a new apartment and expected Tominac to load seven large moving boxes onto his bus for her. He declined.

So you could say Tominac has seen everything but the kitchen sink. Except he saw a Muni rider with one of those just last week.