When Ty had transformed into a young woman, his sperm had not simply been killed by the testicles which they inhabited turning into ovaries. Rather, they had all bound together and been feminized along with the body they were within. In other words, inside Ty was an ovum, ready and waiting to be fertilized.
The sperm which June's body had begun to produce once he transformed into a man, and which he had filled Ty up with, instinctually and eagerly swam towards the egg within Ty's womb. Very soon, they found it and all chewed away at its walls, every single one desperate to get inside it. Any conscious doubts about all of this- how the sperm around this ovum shouldn't exist, the ovum should be a mass of sperm, and the disgusting taboo Ty and June had violated- did not matter anymore.
Soon one of the sperm broke through and reached the centre of the egg, fertilizing it and guiding it to implant in Ty's womb. The perverted desire of both Ty and June had come true, as the girl who had been a boy was impregnated.
Meanwhile, outside Ty's body...