Eli5: Why is it difficult to pee with an erection?

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Typically, when a man has an erection and has to pee, the urine shoots out like a broken sprinkler, but semen usually seems to shoot almost straight. Why?

In: Biology
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There’s an internal “valve” that connects the urethra (pee tube) to either the testes (for semen) or the bladder (for urine), but not both at the same time. For hopefully obvious reasons, you don’t want to mix the two.

Normally, with an erection, it’s set to “semen”. Biologically, we’re not setup to pee with an erection. That’s why it takes a while and conscious effort to pee with an erection, the “valve” needs to reposition. When that happens, you’ve got a different fluid traveling through the erection than it was intended for and the fluid mechanics are all wrong because the tissue around the urethra is still engorged and pushing/distorting the urethra away from it’s normal flacid shape. Basically, the penis is still set for “semen” but you’re running urine through it. Semen is *way* thicker than urine, so it behaves definitely exiting the urethra.

Also, it is bloody difficult to aim when your John Thomas is standing to attention. So you need some time to calm down to be able to do anything other than imitate a fountain.

I don’t really find it difficult to pee with an erection, I find it difficult to pee where I’m supposed to with an erection.

It’s great isn’t it. Every time I have to hold a piss I just think of sexual things (maybe jack it a bit) and baaam, I can hold that shit in for hours.