The prostrate has nerve endings. Insertion of a dildo stimulate them and the stimulation is pleasurable to a lot of men and can enhance the feelings of ejaculation. Since no one is harmed and use of most dildos is harmless, there is no need to worry or feel guilty. Try to let the feelings of shame go but beware that once you do then the fetish may wear off and you may need to find a stimulating new one. In short, if it makes you happy and it is harmless then continue doing it. If the dildo you use is so large or has other properties that could make it dangerous then stop and seek counseling or find a new, safer fetish.