Modern society seems to want to distance itself from the idea that humans are animals, and pretending we don't poop or sweat or reproduce. This is old news. But various recent events have given me the idea that perhaps people have narrowed their focus, and want to distance themselves from mammals in particular. The first intimations of the idea came to me while thinking about the variety of methods and reasons people use to rid themselves of their fur (on their heads, legs, and so forth). I can understand getting outré haircuts, as it's a way to express personality and it always grows back, so no permanent commitment there. And, as someone who keeps his beard trimmed, I can understand shaving (but I loathe stubble). After that, it gets to fashion and personal taste pretty fast.
One of my tried and true ways to explore ideas is to borrow a card from baronmind's deck and employ extensio ad absurdum, the practice of taking an idea to its logical (if ridiculous) conclusion.
What if people tried to remove other aspects of mammalianism from themselves, as either an expression of individuality or just following a trend? One obvious aspect of mammals is mammary glands, but I'm not aware of people having elective mastectomy as a declaration of uniqueness (I'm not counting medical cases due to breast cancer or gender dysphoria here). I'm not saying it hasn't happened (I wouldn't be surprised if it had), just I've not heard of it.
But what about more basic modifications. Mammals are tetrapods, but other than in a John Varley story, I don't recall people having limbs removed to achieve a desired "look" (actually, the space ship captain – a woman named Javelin – removed an arm and the opposite leg so she could zip around her crowded spaceship in zero G with very little maneuvering room). And limbs are fun to have for zero G hugging (as pointed out by the abovementioned Varley, as well as Spider Robinson, Elizabeth Moon, and many others).
Continuing with basic modifications, mammals are vertebrates. What if the next cool thing were to be having your spine removed (this did come up in the Starchild Trilogy, by Frederik Pohl and Jack Williamson, but it wasn't by choice!)? Can you imagine the parties? I'd rather not, thank you.
Another defining characteristic of mammals is warm bloodedness. Perhaps being (literally) cold blooded could become the chic thing to do. I'm not sure how this would be accomplished; externally (via something like Arctic Sun) or biochemically or what. Maybe a cold handshake would become a status symbol. Maybe necrophilia will become (pun intended) cool.