howdy. brms is using ggplot2, so I think what you are asking is a ggplot2 question. Have you tried adding geom_line(aes(linetype=gender)) ?
Edit - maybe you have tried that already. Hmm, not sure. Some data and reproducible example would help, but to be honest, at this point it might be easiest just to create your own data frames with the conditions you want and use fitted() to get the model output and then ggplot2 to plot it yourself.