One way of answering this question would be to point to the way in which marriage between the various characters of this novel actually acts as a foil, as in the majority of cases, marriage places two characters who are opposite in a number of ways together to emphasise their differences and how inappropriate their marriages are.
Let us consider this view by thinking about the main marriage in the novel, which is of course that of Catherine to Edgar Linton. We know that Catherine chooses to marry Edgar not because she feels a visceral connection with him and that she loves him profoundly but actually because she desires the social status that such a union will give her. Although Nelly Dean informs us that the first few years of their marriage are uneventful and tranquil, once Heathcliff returns, it is clear that Catherine's wild passions and ungovernable tempers are resurrected, and that, by contrast, Edgar's mild-mannered and softly-spoken approach to the world are shown to be weak and ineffectual.
In the same way, if we consider the marriage of Isabella to Heathcliff, we have youthful naivety and innocence on the one hand balanced with experienced cunning and manipulative tyranny on the other hand. The marriages serve to throw two opposites together, forcing us to relate the marriages to the overarching conflict of the novel as nature (as symbolised in Wuthering Heights and its residents) is pitted against civilisation (as symbolised in Thrushcross Grange and those who dwell there).
One of the few marriages in this novel that does not involve conflict is of course the marriage between Catherine the younger and Hareton, and it is highly significant that they will leave Wuthering Heights and live in Thrushcross Grange together. This symbolically indicates that they are marrying as equals who will be able to lead a happy life together in society, having left behind the ungovernable passions and nature that characterised the relationships of those who went before them.