Ah, yes, the 45-minute love affair. Very big in the 50s and 60s, where you didn’t have ongoing plot lines: your hero would fall in love, but at the end of the episode, he would end up alone – either she was dead, or they decided to go their separate ways.
James T. Kirk had a few of these, most notably with Edith Keeler.
Is it still done? I think so, though usually the affair takes a long time. Consider Andy Sipowitz in NYPD Blue: he fell in love, married, and saw his wife die over the course of a few seasons. NYPD Blue also did this with Kim Delaney, who had Jimmy Smits die on her (whenever the writers of NYPD Blue were stuck, one of the main characters died; I would not want to be too close to Andy Sipowitz, who had one son, one (maybe two) wives, and a partner die on him).
Horatio Caine of CSI: Miami has his new wife killed this season, so the practice is alive and well. It’s just stretched out over a season instead of stuffed into one episode.