Yeah, ever since Willow and Tara...
Here's my experience, take it as you will: Romantic Subplots are not required, but the entertainment industry seems to THINK they are. Books and movies tend to get farther if there is a romantic subplot. I suppose this is because it engages the emotions, particularly of female readers, and some genres which are principally aimed at men may attract more readership this way.
In "romance" genre books that aren't specifically erotica, there will be two and a half sex scenes. By this I mean that two scenes will be fully (if ambiguously) described, and another will be alone the lines of "then they fell passionately into bed", the end. Note: if there is more than one pairing in the book, this formula changes.
In books that are not of the romance genre but contain a romantic subplot, there will usually be one sex scene and possibly another 'half' sex scene.
In books that are not of the romance genre but contain an UNIMPORTANT romantic subplot, the pair may share a kiss and possibly some groping, but may or may not have sex within the confines of the book.
Those obviously aren't absolute rules, just patterns I've observed. You may or may not agree with my interpretation of them, but if you're trying to decide how important romance is to your book, it might be helpful to have a relative measurement?