Aliens are having sex with humans. Don't beleve? Read This:
In 1957, a 23 year-old Brazilian farmer named Antonio Villas Boas and his brother observed a strange red light in the night sky. The brothers thought it unusual but dismissed it as something and nothing. The following night they noticed it again but again they thought it was interesting but nothing more.

It was a few days later when working in the fields on his tractor that Jose was to discover first hand the origin of the strange red light. To his amazement, a saucer shaped craft landed close to the field where he was working. Terrified, Boas tried to drive away, but as the craft touched down the engine on his tractor mysteriously failed. Boas attempted to run but found himself unable to, the aliens seemingly exerting some kind of control over him. He was apprehended by a group of humanoid aliens, which he describes as being about 4 feet tall. Despite their stature they were easily able to subdue the terrified man and take him aboard their ship.

Once on board, he was stripped naked and his whole body covered with a gel of some kind. Two of the aliens took a blood sample from his chin and left. He noticed puffs of smoke or vapour that made him sick to his stomach, coming from vents in the wall, which caused him to vomit. After a while what he describes as a ‘beautiful’ female alien came into the examination room and made it clear that she wanted to have sex with him. Boas had little choice but to comply although he described the alien as being very clinical, without any feeling or emotion. Once she had been successfully impregnated, she pointed to her abdomen and then the sky, which Boas interpreted as meaning that she would give birth to the hybrid child in space.

Boas was then given a tour of the ship, but said that the aliens were generally emotionless and not forthcoming or particularly friendly, before being released. Still in a state of shock, he watched as the craft shot off skyward. Reports of Boas’ encounter was generally kept out of the press due to the explicit sexual nature of the contact, which wasn’t considered suitable for publication at the time. In time Boas married, had a family of his own and became a lawyer, right up until his death in 1992, he maintained that his abduction experience was genuine and that somewhere out there is a hybrid child that he fathered. LINK

How do aliens reproduce?