has no one seen those movies where guys wake up with an erection and pee on themselves? 40 year old virgin, anyone? it's a funny thing to laugh at because it's true. you can definitely pee with an erection, but it's just more difficult to do because you're peeing up. if you have to pee and you're having sex and about to ejaculate, your bladder sphincter closes off to allow for ejactulate/semen before it will let you pee.

and for the anon making a googly eyed comment below, because i'm a pedant, there are multiple 'sphincters' in the body. it's a term for a ring of muscle that closes off around something. there's a sphincter up near your diaphragm/stomach. sphincter is not unilaterally owned by the anus.

(sorry, but i feel i have to chime in on this because so many people don't know basic biology about their own bodies: having to discuss with grown adults that there's three holes down there for women and two for men and that you don't piss out your vagina and you can't 'hold in' your period. it's frustrating.)