"Humans weren't always like we are now. We used to have a spherical body, 2 heads facing opposite, 2 pairs of arms, 2 pairs of legs."
"There were three combinations of sexes. Two males who came from the Sun, two females who came from the Earth, a man and a female who came from the Moon."
The 2 male bodies split and became gay, the 2 female bodies split and became lesbian, the 1 male and 1 female bodies split and became heterosexual.
Over time, they became presumptuous and tried to take on the gods.
This angered Zeus and he decided to chop them in half to make them weaker.
After sometime, Zeus saw that the humans were struggling and saddened. He felt pity for them. so, he fixed them up from hemisphere-humans to what we look like today. The humans learned to accept their lives. Since then, when a human finds their other half, they never want to separate.