I don't think Sei has an Oedipus complex. I think his mom pops up in his mind when he feels aroused because he was groomed affectively. His only sexual encounter/knowledge before the kiss was his mother's grooming. So it's not surprising that she lingers in his mind during sexual moments in his life. This is one of the main reasons victims of incest have trouble talking about their trauma. Because even tho all victit's of sexual trauma can have moments like this, the abuser being a family member muddied the waters and make you look complicent.