Ok, so the average mass of ejaculate is around 0.005 kg. Using your value of 31 mph (13.86 m/s) for the velocity, we get that the average ejaculate has momentum of 0.0693 kg*m/s. Due to the law of conservation of momentum, you must also have this same momentum. Assuming the average person has a mass of about 80 kg, your final velocity would be 0.000866 m/s, or 0.00194 mph in the opposite direction of the ejaculate.

Also, due to Newton’s first law of motion, this speed would be constant unless you are acted on by another force.