They concede too much.
When you defend a dark YA novel because darkness in YA novels can teach empathy and awareness for various social problems, you make that the standard for judgment--rather than saying, hey, you know what? Teens are people, and novels aimed at teens deserve as much breadth and as much of the author's critical and artistic judgment as any other art form aimed at people. Period. Sure, you try to figure out what your audience will get and what they'll like--you always do. That's true of any novel.
And when you defend genre romance by noting that a great many writers--usually Jane Austen comes up here, often Shakespeare--have concerned themselves with love stories, you are palming cards that only readers who are sympathetic to your cause will miss: the idea that a love story and a genre romance are identical, rather than the latter being a subset of the former. Nobody actually buys the, "One genius could make this a worthy thing, so it's a worthy thing!" argument, particularly not the sort of people who are condemning entire genres wholesale. They buy them even less when the worthy thing you're pointing at is arguably not the thing you're defending. Shakespeare's genre, in the old technical sense, was plays or poems, not novels. If you have to call on Shakespeare to defend a group of novels, you have already lost your argument nine times out of ten--worse, you have conceded it by not forming a coherent defense of what you actually value.
When you rely on moral improvement or references to the canon to defend what you like, you are reinforcing the idea that these are the standards on which these things are to be judged. And that's far too narrow a view of fiction. "You're being unnecessarily prescriptionist," for example, is a reasonable thing to say, or, "Not every book has to do the same thing, and here's what this one does well," or even, "Go to hell; I like books like that." But when you're defending genre romances, for heaven's sake use genre romances to do so. When you're defending SF, you don't have to use that one time that totally reputable writer wrote one book that's sort of SF except totally not in conversation with the rest of the genre. You can say, "Here's what's lyrical in this book," or, "Here's what I found interesting," or relaxing, or touching, or fun. Some of us have fun with dark stories. Some of us have fun with a particular genre type of love stories. Some of us have fun with books that are intensely focused on language or theme. Fun is okay. Do not concede the fun. It doesn't have to be everybody's fun, but it can be yours. It doesn't have to be Morally Uplifting or Deeply Intellectual--some books are, and that's great, and some aren't, and that can be great in its own way too.
Incidentally, it's also okay to not bother to defend what you love and what you do when people are sneering at it ignorantly. I'm glad that sometimes people do so, but it's also sometimes okay to look at those people and say, "Okay, but moving on, I want to talk about something more interesting now because we have done this and that person was clearly not listening." People will keep sneering at romances. They will keep sneering at literary fiction. They will keep sneering at SF. It's not because you've been insufficiently eloquent to silence them. It's that they have a shtick. You are not required to care about their shtick.