A new edge-based sexually transmitted SEIR model on the contact network is introduced in this paper. The contact infection between the opposite sex and no infectivity during the latent period on bipartite networks are included. The basic reproduction number and the equations of the final size of epidemic are derived. The dynamics of our model with arbitrary initial conditions are further studied. Sensitivity analysis on several parameters and numerical results of the model are derived. We show that the length of the latent period has an effect on arrival time and size of disease peak, but does not affect the final epidemic size and the basic reproduction number of the disease.