Quite simply, when imagining another person during sexual act / intercourse, the imagination becomes a reality. In other words, there is no longer an emotional bond with the sexual partner. Instead the sexual partner is replaced mentally by imaginary person. By sexual fantasying sexual desire, curiosity and intensity will all be multiplied. But unfortunately emotional bond and spiritual intimacy is lost. No problem if the sexual fantasy happens occasionally. If sexual fantasying happens again and again and becomes a habit, it can become a real problem in sex life.
If sexual fantasy becomes a habit, the sexual partner may realize it at some point. Gradually their sexual interest fades. Sexual acts will be devoid of any emotional involvement. Sex becomes a just bodily physical activity without any psychological and affectionate bond. In due course sexual life will be lost.