How many TV shows have done better than “The Big Bang Theory” with long-term relationships?

Perhaps they won’t go down in history with Jim and Pam, Maddie and David, Sam and Diane or Beatrice and Benedick, but with Valentine’s Day approaching, it should be noted that the CBS sitcom has kept things lively with no fewer than three major ongoing romances among its characters.

The topper has been the love story brewing since Day 1 between Leonard (Johnny Galecki) and Penny (Kaley Cuoco, taken for granted by the TV Academy and overdue for an Emmy nom). The two have been circling each other for 5 1/2 seasons now, occasionally dating other people but always the series’ romantic focal point.

But in recent years, the building connection between Sheldon (Jim Parsons) and Amy (Mayim Bialik) has been a treat. Each character was nearly robotic at conception when it came to social skills, which makes it all the more remarkable how naturally their intimacy with each other has evolved. A couple years ago, you wouldn’t have dreamed it possible for Sheldon to be in any kind of close relationship.

Throw in the solid love affair-turned-marriage of Howard (Simon Helberg) and Bernadette (Melissa Rauch), and you’ve got three couples defying the conventional wisdom that love is the death of comedy.

BBTLast week’s episode offered the most recent example of the fine touch “Bang” has had, with intersecting stories involving whether Leonard-Penny and Sheldon-Amy would start living together as couples. In each, one of the characters (Penny and Sheldon) were able to show how torn they were about the relationship moving too fast for them, without it feeling contrived.

But the finest moment of the season might have been December episode “The Fish Guts Replacement,” when Amy became ill, but realized that the TLC she would be getting from Sheldon would help break down the barriers against physical contact they’ve had.

With its ever-booming ratings, “Big Bang” has a long way to go and might someday stumble. But for now — and for a long time now — the show has given a master class on how to make romance on TV work.