Description:The penis, which usually hangs limp, becomes hard when a male is sexually excited. Tissues in the penis fill with blood and it becomes stiff and erect an erection. The rigidity of the erect penis makes it easier to insert into the female s vagina during sexual intercourse. When the erect penis is stimulated, muscles around the reproductive organs contract and force the semen through the duct system and urethra. Semen is pushed out of the male s body through his urethra this process is called ejaculation. Each time a guy ejaculates, it can contain up to million sperm.