"I have espoused you to one husband"
= Paul has engaged the Church to be wedded to their future husband, Christ.
"that I may present you as a chaste virgin to Christ"
= Since the Church has been engaged to wed Christ, Paul seeks to keep the Church as pure from sin as possible.
At the time Paul was writing, the Church had not yet been married to Christ. It was engaged to marry Christ, but the wedding had not yet taken place. So, Paul was encouraging the members of the Church to remain faithful to their future husband and to not sin, lest they be led away from him.