There's a few reasons that Edward desires Bella's hand in marriage. Well, Edward is an 'oldschoolled gentleman' and during his 17 years as a human, he was taught the traditional principles of marriage. So marrying Bella was a very important priority for him, that of his human life. Also, Edward is definitely not fond of Bella being turned into a vampire, especially by him. He does not want her to give away her human life. As a result, Edward insists that he will only turn Bella into a vampire under the condition that she marry him, one of his own desires of his human life. Another reason is because he is absolutely against Bella's human life ending. He is very sad that she wont get to live through all the experiences life has to offer, including that of marriage. So he wanted to marry her so that she could achieve one of those experiences.