Family basically disowning and kicking them out (this includes teenagers), honour killings (where the male relatives kill the daughter that refuses to marry the man they've picked out for her), acid attacks and other harm (such as physical violence), mistreatment, and even kidnapping (a "holiday" to Pakistan, for example, results in her being kept there against her will, sometimes forcibly married to someone old enough to be her father, and her passport taken/destroyed).
Fucking hell, god help them if they're a Muslim dating a Hindu, for example -- one of the actresses who played one of the Patil Twins in the Harry Potter films was assaulted by her family because her then-boyfriend was Hindu (she was raised in the UK but her family were Bangladeshi-Muslim) -- jokes on them, though! She disowned them, got away from them, married the guy, and now has a kid with him. Good on her.
But yeah, it's far more common with girls than boys.