In my own personal headcanon, because we know that Marietta's mum was helping patrol the fireplaces at Hogwarts, Marietta knew that Harry had been in contact with Sirius Black. And since the trio/Order were the only ones who knew he was actually innocent at the time, everything got ratcheted up from "my best friend is dating a jerk who will get us all in trouble" to "my best friend is dating a jerk who is in cahoots with a mass murderer!"
It doesn't make what Marietta did right, of course, but it adds another level of fear/urgency/I haven't had my coffee yet so I can't think of the exact word I want but it made it more of a big deal in her mind than just petty teenage stuff.