Is marriage obligatory for a girl who fears falling into indecency due to watching pornography and masturbating, given her father's unlawful refusal to marry her off?

You must cut off the avenues to corruption and desires by refraining from watching websites that incite them. One who is unable to marry is commanded to be chaste and to turn himself away from incitements. It is not permissible for a guardian to prevent a woman from marrying a suitable man under the pretext of completing her studies. Marriage, for one who fears temptation, is obligatory and takes precedence over studies. Try to come to an understanding with your mother to persuade your father, or ask her to seek assistance from people of sound judgment and wisdom, or the imam of the mosque. If he does not agree, then be patient and remain chaste until Allah facilitates your matter.