What's to be ashamed of?

My utopia is one where, even though people still have monogamous relationships if they want to, and take all the time they like building up to being sexual with people, no-one feels any need to hide their body or their sexuality from others, so public nudity and sex are everyday normal occurrences.

But my hottest fantasies, including this one, take place in a hybrid world where the people in the story are totally unabashed but the social taboo is still in place. In this fantasy I don't even have sex at all, I'm just on a train and there's a guy sitting on his own. A woman gets on and they're clearly partners because they hug and kiss and talk about domestic stuff. They sit pressed up next to each other even though it's a seat that would fit three, and they're just enjoying being in each other's physical presence at the moment. They kiss and laugh and ignore everyone else on the train. 

They carry on kissing and start to get a bit more sensual about it. The man puts his arm around the woman's waist and hugs her gently towards him. She puts her hand on his knee and runs it slowly up his thigh. They gradually get hotter and hornier, each one testing the consent of the other before taking things to the next level. At one point the woman has her back against the man, her hand is in his crotch, pretty clearly rubbing against his cock. He has his hands under her jumper rubbing her breasts. 

They're making an effort to be quiet, just the way people would if they notice their conversation is getting a bit too raucous for everyone else on the train. They're not embarrassed or shy at all, but they don't seem like they're making a performance of it either. In the end the woman's bra and underwear is on the seat beside her, her skirt is up, her legs are open and she has an orgasm while her boyfriend and herself both finger her clit and cunt. His pants are on the floor in front of him and he cums as she rubs his cock, catching his cum in her mouth.