My semen seems to come out when it's not erect or if i do not masturbate a long time. is there a way to stop this from happening?

No, but coping...: We are not born equal in the anatomical detailed features of any organs among all individuals. What you reported merely indicates the likelihood of poor sphincteric function of ejaculatory duct & seminal vesicles, which would have no cure, but need to cope with it by avoiding constipation, straining to defecate or urinate, using protecting pad, and it is okay/harmless as long as being functional..