The best reason anyone could give for birth control whilst making love with a bull, particularly if you've enlisted the help of a known genius.
Minos, king of Crete, decides that he's going to make a sacrifice to Poseidon. He sets up for the sacrifice, but, being somewhat of a mooch, decides to ask Poseidon for a bull. I mean, this guy's king, no? He's got plenty of cow wandering Crete, but he asks the god of the sea for one--it's not as if there are vast bovine herds grazing on the kelp down there. Anyway, Poseidon sends a beautiful white bull out of the sea. Minos likes this bull so much that he decides that it'd be a shame to kill it, so he goes and gets one of his own cows to sacrifice.
Poseidon isn't too thrilled with this. It's bad form to ask the gods to provide their own sacrifices, but it's spectacularly bad form to decide to steal the sacrifice for yourself. These are Greek gods we're talking about, not piddly-assed dilettantes lounging around, delivering bulls like some obscenely obsessed UPS driver. Greek gods take revenge. And not in the Judeo-Christian tradition, striking people dead or causing the earth to swallow them. No, they're much nastier. Poseidon causes Minos's wife Pasiphaë to fall in love with the bull.
Now, this would have been embarrassing in and of itself, but Pasiphaë's got a friend named Daedalus who's sort of the Leonardo da Vinci of the day. Pasiphaë needs to convince the less than amorous bull to have his way with her, but she's not really his type, so she turns to Daedalus. Daedalus is always up for a challenge, so he built a Trojan cow she could climb inside. He was obviously a good enough artist to fool Poseidon's bull, because nine months later the queen gives birth.
Minos probably wasn't all that thrilled when he got his little bundle of joy, since the paternal resemblance was pretty strong and this was one of those times when you really didn't want to have daddy's eyes. But it was a member of the royal family and Minos couldn't just off the kid, so he went to the Oracle at Delphi who suggested that he get Daedalus to build a cage for the Minotaur. Daedalus designed the labyrinth, stuck the Minotaur in the middle, and Minos dropped helpless youngsters in every once in a while as Minotaur snacks.
This was kind of hard on the locals, so upon winning a war with Athens (the Athenians, under the command of their king Aegeus, had killed off Minos's son, Androgeus, and Minos had taken revenge with a little help from Zeus) the terms of peace included seven lovely maidens and seven lovely youths to be delivered every nine years and fed to Mr. Minotaur. Athens was less than happy about this particular arrangement.
Some years later Theseus, son of Aegeus through a series of complicated circumstances, decided that it was time to do something about this tribute. He joined the thirteen other lucky Athenians and sailed to Crete, where an amazing display of aquatic daring (He retrieved the signet ring of Minos from the bottom of the harbor with a little help from friendly dolphins and Nereids) convinced Minos's daughter Ariadne that he was something special. She went to Daedalus and asked him for a little help for Theseus. Daedalus, never one to miss an opportunity to show off by solving a problem, especially one he had created himself, suggested that she provide Theseus with a ball of string to help him find his way out of the labyrinth. She does him one better and provides not only string but a sword.
Theseus goes in and finds the Minotaur. The Minotaur, being unused to heroes with big swords, is easily finished off, and Theseus follow the string back out of the labyrinth where he is greeted by Ariadne, with whom he runs away with to live happily ever after by Greek mythology standards. Which means he abandons her on an island, she curses him, and Theseus's father jumps off a cliff and dies.
As a side note, when Minos finds out Daedalus's part in the slaying of the Minotaur, he imprisions Daedalus in the labyrinth with Daedalus's son, Icarus. But that's another story.