Leto was a cousin of Zeus – the daughter of his uncle Coeus and aunt Phoebe, Titans like his own parents. Not that this – or indeed, anything else – would have stopped the god of sleeping with anyone, anytime. Unusually, he didn’t take the form of anything on this occasion – he was just his godly self.
Hera, as usual, was unimpressed, and also as usual, took it out on the woman rather than her husband. She proclaimed that Leto would not be allowed to give birth on “terra firma”, the mainland, any island at sea, or any place under the sun. She eventually gave birth to the twins Artemis and Apollo on the isle of Delos.