Well, since you brought up dark Serge, I have one more question.
did it count as incest when Lynx stole Serge's body?
And the reason why people choose Magus over dark Serge: Magus has a story, a personality, and motives. Lynx/dark Serge is just a puppet. True, he has more story than most people in Cross, but he has no motives or personality of his own, only what FATE gave him (so he's already taken by two women; his old wife, and FATE).