There was no good reason for Cosette and Eponine to fall in love with him.
Well, there actually are good reasons, you just have to read the book to find them. Cosette and Marius were both secluded people, and for that reason they unconsciously identified with each other. Plus, Marius also wrote her a very sweet and romantic letter, and you know how much Cosette loves stuff like that. :) Eponine is a little more complicated. She fell in love with him because he was the first man who had treated her with basic respect and courtesy in a really long time. It didn't matter that he barely even knew her and vice versa, because he was kind to her. So Eponine clung to that like a lifeline.
Marius is a jerk!
Marius does not have the most appealing personality. He's maybe a little self-centered, but name me the perfect Les Miserables character. At his heart, Marius is a decent man. Yeah, he kicked Jean Valjean out of his house because he thought he had murdered a guy. But when Marius learned of his mistake, he hurried to make amends as fast as he could. I'd say that Marius' redeeming quality is his gallantry. Even when he doesn't like a person, he's relatively courteous to them, and that's something that's really hard, and at least a little admirable.
Marius is a dork.
What? Is it because he's indecisive? Because he's socially awkward? Because he doesn't look like some archangel who fell from heaven? I suppose by those standards Marius is kind of a dork, but he's also relatable, and don't we love characters who are 'relatable'?
Yes. Marius is stubborn, tactless, and short-tempered, but he's a decent person, and he can break his curse on his own! Oops, slipped into Howl's Moving Castle mode again. That'll happen. But you get my drift.
Something I really liked about Les Miserables is that you have characters all over the map when it comes to morality status. You can have 'good guys' who are unpleasant and a little rude (Like Marius), and you can have sympathetic 'bad guys' with understandable motivation (Like Javert). Then there's some characters you can't even classify (Like Eponine or Enjolras). It's very realistic, and definitely one of the book's strong points.
Stay frosty, my friends.