Backwards City points to the top 10 reasons sex at the speed of light is not an advisable form of procreation:
7. Relativistic flaming semen:
In the unlikely event that a vaporised penis can perform ejaculation, then the relativistic semen will create enormous air resistance, burst into flames almost instantaneously, and generate enormous impact forces. These forces will be sufficient to pierce a small hole straight through a woman's lower torso, just like a speeding bullet, only incinerating the surrounding tissue as it passes through.
"Relativistic Flaming Semen"... great band.