One thing to remember is that rules of propriety were different in Shakespeare's day than many people hold them to be today. In Shakespeare's day, a young unmarried woman remaining chaste was considered a virtue. Audrey may be a simple, uneducated country girl, but she evidently very much prides herself in her virtuous chastity and is not likely to relinquish her virtue very easily. What's more, Touchstone is really only interested in a relationship with Audrey for the sake of satisfying his lustful desires. If he could get her to agree to yield her virtue without marrying her, he would probably do just that. However, he knows very well that, being the virtuous country girl she is, she absolutely will not yield her virtue unless she is married; hence, Touchstone devises a scheme to marry her.
We especially see Audrey proclaim her virtue in the first scene in which he courts her, Act 3, Scene 3. When Touchstone tries to wile her with his educated, witty tongue and she fails to understand, he then states he wishes she was more "poetical," and Audrey refers to her virtuous nature in her reply:
I do not know what "poetical" is: is it honest in deed and word? is it a true thing? (17-18)
In these lines, the word "honest" has a double meaning. In one sense, she is referring to being "honorable and true," but a chaste woman is also considered an "honest" woman (Shakespeare Navigators); hence, with just these two simple lines, Audrey shows how much she values all of her honorable virtues, especially her chastity.
We also learn about Touchstone's devious plan to marry her and leave her in this same scene. Jaques comes forward and cautions Touchstone not marry her under the clergyman Sir Oliver as the marriage may not be fully legally binding. Touchstone's reply is to say he doesn't mind if the marriage is not legal as it would make it easier for him to leave her. Yet we must question his sincerity in saying he does not want to legitimately marry her, for next he marries her, along with the other couples, under Hymen, the Greek god of marriage ceremonies, which they certainly would consider to be a very binding marriage. Hence, while it is probably true that Touchstone is marrying Audrey to satisfy his lust, it's unlikely that he really will leave her in the end.
Maybe too late to answer now, but its mostly because Touchstone feels that he needs entertainment while he is staying in the forest and that once he no longer wanted Audrey, he could just easily leave her. Audrey, being a simple woman, would not protest much. Their love is an example of sexual love, unlike Rosalind-Orlando's and Celia-Oliver's true love at first sight.
It may be a little wrong.