As a scientist, there are two primary flaws with any ejaculation-based propulsion plan:
1) Even being immortal, there is no guarantee that you can perpetually ejaculate into the cosmos without replenishing yourself with the water and nutrients required for your propulsion system.
But since we are already assuming immortality, let us also assume an infinite supply of propulsion. This brings us to the second and much more serious issue:
2) Velocity vector – in general, if you point your urethra in the same general direction each time, you will indeed build up thrust. However, it would be best to orient yourself at a specific star.
Given Newton’s Third Law, that every action requires an equal and opposite reaction, you would effectively be launching tiny streams of cum at an entire solar system.
As the eons pass, your speed builds, but eventually it requires a nearly infinite amount of ejaculate to push yourself only fractionally closer to lightspeed.
This means a nearly infinite amount of cum would rain down upon the planets of whatever unfortunate star system you selected. Given that the volume of your ejected propulsion is infinite, gravity and orbital mechanics guarantee that entire planetary systems would, eventually, become engulfed inside gigantic globules of your gossamer wads.
Incidentally, some scientists theorize this is how Pluto was formed.