Wen Ruohan was expanding his sect as much as he could, wanting to take over the entire cultivation world. But, realizing Gusu Lan's importance, he then proposed an arranged marriage. One of his sons would marry Lan Wangji. That's how you came to Gusu, in the red wedding robes. The Lan sect, mainly Qiren, didn't want this marriage, but they didn't have any other choice. And Lan Wangji, as much as he harbored feelings for Wei Wuxian, was content to marry, for the sake of peace between the sects