From what I am able to gather, it's simply because There are 2 sets of sphincter muscles: internal and external sphincter muscle of urethra. During ejaculation, The inner sphincter muscle mass closes to prevent semen from entering the bladder. Hi there. Referring towards your problem of passing thick sperm http://reidplhat.isblog.net/i-wee-when-i-orgasm-can-be-fun-for-anyone-13203687