5. Lack of visual appeal:
Time-dilated necrophilia, flaming relativistic ejaculation and penile black hole formation are all very dramatic, but unfortunately they don't translate well onto the big screen. In reality, relativistic sex would only last for a fraction of a second, and would appear as a sort of muddy grayish white smudge, since the eye merges all images together at such high speeds. This is probably not visually appealing enough to make a porn-at-the-speed-of-light series out of.