Not only are they literature, when you put a good romantic story up against the likes of some depression phobic like Steinbeck, it's clear why they're so popular. They take you away to a better place. They show time between two people who are perfect for each other but life is keeping them apart. After overcoming impossible odds, they find true love, which is the way it should be. It makes a reader feel good inside. This is literature at its best.
In general, romance novels are literature; just because they're romance novels doesn't make them 'worse' or 'not literature'. However, some modern romance novels, particularly the ones with nothing BUT romance/love etc. in them...not so much, in my opinion. I honestly can't see any way Twilight and Fifty Shades of Grey can be considered literature.
Too many people decide that things they do not like are worthless. I am not a fan of romantic literature, but to say that it's not literature is arrogant, as though my opinion is fact, which is ridiculous. Regardless as to your feelings about romantic literature, a lot of people love it, and it is relevant to them. This makes it legitimate.
Romance novels are mass marketed books intended to make a profit but they are still the written word. History as it always has will determine if its literature or just pornographic rubbish but either title may not be permanent as society evolves so does its definition of modern and historic works.
Romance novels tell a story and reveal the human condition just like any other literature. Love is one of the most powerful emotions humans can feel and romance novels often contain an element of emotional truth other works do not. Genre is such a limiting tool anyway, romance novels are literature and should be considered as such.
It would be like calling a reality television program a film or an Instgram selfie taken in front of a mirror with an iphone photography. Literature should be evocative and intellectually and or emotionally provocative. It should change you in some way, force you to think challenging thoughts. Reality TV is media, but it is not a film. Instagram selfies are pictures, but they are not photography. Romance Novels are a form of writing, but they are not literature.
Classic romance stories from respected sources like the Bible or Shakespeare are considered substantial literature. However, books that simply tell a fictional story of a woman having a chance encounter with a devastatingly handsome man can't be considered literature, in my opinion. The reads may be entertaining, but they shouldn't be in the same category as other, more sophisticated literature.