The Romance Novel community gets quite defensive about the "softcore porn" comparison. And rightly so, since it's not all about the sex.
Whilst Pillywiggin and I were being led around Italy on a bus by an Austrian Legolas (this did not go well) I explained my ideas to her and we joked on a number of studies we could try doing (us both being larval academics). She recently shared this article, "Mate selection in popular women’s fiction," which is fascinating and thoughts on that pending (it has a generator table for romance novel plots!).
But first, onwards to my heresy.
Let me first explain parental investment theory. It's nothing particularly new, or particularly controversial, as far as I'm aware it's as follows:
Reproduction requires a high level of investment for a woman: there's the metabolically costly egg, not to mention nine months of it growing in her and another year or more of breast-feeding. On the other hand, for men, what's required of them is one act of fruitful intercourse.
So women desire strong relationships from their sexual partners such that they'll stick around and look after the child (and herself) after the act. All their eggs are in one basket, so to speak. But men try to have as much sex as possible, which is low investment and low commitment, which spreads their genes far and wide.
Now, what I'm proposing is this: romance novels and porn are related, or rather analogous, in that each plays out the aforementioned "goal" of the corresponding gender. Romance novels aren't about sex, they're about the development and maintenance of stable romantic relationships, often ones that result in children. And thus the happy ending is a must, because the relationship must be successful. In psychology terms: mate-selection and mating commitment. Equal and opposite to it is porn, which is about the sex, as much sex as possible, with fertile-looking women. All the selection, none of the commitment.
From this, I'm not trying to say all men want sex and all women don't. People are more complex than such a study with looks at only an aspect of these things can encompass.
And certainly, not all romance novels fall neatly into this paradigm (I'm still trying to work out where gay and lesbian romance novels fit in; slash I'd suggest fits into the relationship-building part for a female audience, but beyond that I'm not so sure; Family saga novels which correspond to romance novels to some degree - said to be more popular than romance novels with women in the UK - can also be read within this model.)
(I'm also in no way saying romance novels are porn, so please don't flame me.)