I think it is commonly seen as sexy due to advertising which intentionally tried to portray smoking that way.

Particularly in the 1960s smoking advertisements started to target women consumers, like with the Virginia Slims advert, and as smoking went from a strictly masculine activity to something liberated women do, I think the door opened for adverts to increasingly associate smoking with sexual appeal.

However, smoking a cigarette after sex is its own phenomenon, associated with relaxing after sex but it’s probably more about the fact that smokers need their nicotine fix after having not smoked for an extended period of time.