You can't get an erection in space.
I expect that's not true.
If the normal blood flow and pressure through my body required to keep me alive still works, then I can't see why other functions of blood pressure would not also continue to work.
All you lose is gravity, and the loss of h-rho-g hydrostatic head.
But if you are on Earth, lying flat on a bed, the h term in the equation becomes zero, and so g becomes irrelevant.
The pump ( heart ) still provides sufficient head for the purpose.
Now, if you were thrown out indo deep space ( perhaps after listening to Vogon poetry ), the pressure differential may indeed enhance your erection as in the manner of the manhood-enhancing vacuum pumps offered on the Internet (or is that just my Internet? ).
Anyways, enough of this...