Depends on what you mean by sexual intercourse! Alec and Bertrand mutually consider their first time, despite some prior experimenting, to be the blowjob Alec gave Bertrand and the handjob Bertrand gave Alec when they were twelve. It's the first time either of them made the other come, and it took place in what was nominally Bertrand's bedroom but usually shared by him. Bertrand saw the encounter as coercive on his part, thinking he was taking advantage of Alec, but Alec had wanted it for months and had been practicing. Both of them left the encounter elated.

As for their first anal sex, if that's what you consider intercourse, it's a matter of debate between them whether they were fourteen or fifteen. It happened in their dorm room at Eton, and Bertrand topped using Vaseline for lube. Again, elation. While their sex life was damaging to both of them in deeper ways, neither of them really detected it, too busy focused on the feelings of euphoria they could chase together.