The marriage would be considered lawful with out their consent. But before you rush into marriage without their approval you should make sure that the reasons of their objection are invalid ones. Because otherwise it would be wrong. If you're marrying someone whom your parents don't approve of her character then it wouldn't be permissible to marry her because she has a major flaw. If for instance your parents know very well; completely certain that she is not a good woman or she is unqualified as a wife and she is doesn't care the least bit about Allah then ofcourse this kind of marriage would be highly discouraged. If your parents reject her because of invalid reasons like her background or her being disabled or their being too much differences..etc and you know that you can handle those things and she is most importantly a good person I think it is lawfull to marry her. Your parents will come around God willing. If she is a pious person she will respect them as she respects her own parents and maybe that will soften their hearts towards her.