It was the conversation between Dylan and Sam in his bedroom when they saw Sophia posing in lingerie for Liam, and she listened in on their chat. It is when you could say that Dylan started the idea to pursue Sophia. You can say that Sam gave him the idea for it.
In the conversation with Sam, Dylan always struck me more like a person who has been caught with homosexual tendencies, but who would rather shoot the witness than admit that the witness is right. He really denied any unmeasured interest in his mother.
Dylan discovered very early on, as a child, that his mother was also a sexual person. I think that's one reason for his sexual interest in his mother. But I also believe that this conversation with Sam was not the first of its kind in young Dylan's life. This is pure speculation, but it would seem normal to me that Dylan, as the son of such a desirable woman, even in his old home, would have been the target of the very taunts and jokes from classmates and friends that he mentions in the hot tub at the last event.
So for me it's not a sudden switch but a long, slow process in which these jibes have repeatedly pushed Dylan in this direction.
