If you are having sex with one person while thinking of another on a regular basis then to put it frankly you are with the wrong person. What you are really doing is scattering your sexual energy.
Thoughts like everything else have energetic blueprints. When you think of someone else your energy is going to that person and that person at the same time comes closer to you. This is because we are always looking at different facets of ourself. Wherever we have put those aspects of ourself as separate we will be looking to others.
An example of this is the man or woman who is looking to the opposite sex to fulfill the role of the masculine or feminine principle for them. When this is seen as separate from them then what they are doing is creating an energetic attachment to that person that represents that principle for them. This also goes for anyone that does this with any person in their life.
Also when having sex with someone you are mixing energies with them for both of you are made up of more than just your physical bodies. Each of you has thought forms and experiences that have shaped your bodies and brought you to where you are in your life. This is also what you are mixing with when you have sex with a partner.
If you are thinking of one person when having sex with someone else then you are not really in a partnership with yourself or the one you are having sex with. The one you are having sex with is in the same position. This is the key to why so many relationships split up.
There are aspects of who we are that can act independently of our outer consciousness. If it's your personality choosing a type of partner then this is what happens. Everything in your aura, chakra and body systems is evolving or degenerating according to where you are living your life from.
If you know who you are intellectually or on more of a spiritual level of being but are not following or integrating those levels of being with your physical self then what you will be doing unconsciously is creating an entanglement or what could be called an etheric triangle.
This is what happens increasingly in this world because we are in times of accelerated change. We must become more identified with our spiritual self, integrating our Soul Light Design in this world. If we are just feeding a relationship from our physical being without engaging the relationship with our own soul's blueprint then what is happening is the people doing this just parrot every body else. Their unique voice in the world gets lost or enmeshed with a partner.
What is actually happening is psychically people are tapping into someone's mental, and emotional bodies and other levels of being that are needed to be the mate of a sexual partner. To put it simply a physical mate is not necessarily a spiritual mate of someone.
If we have a split here in awareness then we can create disease when having sex with someone depending on how much we have integrated our more refined spiritual energies with our physical body. This is where being scattered in our energies by thinking of someone else can be detrimental to our physical being. People unconsciously can be doing things energetically that they are not aware of with their outer personality.
For example if a person is jealous of someone else that their partner may be attracted to they can be unconsciously trying to block the connection even to the point of trying to harm them. It is only our body that is of this lifetime. Our consciousness is multi - dimensional. If the person on the outer world awareness is afraid to dig deeper on an inner level of being they can be dumping their unresolved issues on a partner when they have sex together. This stuff clouds up the energy field of both people as well as whoever either partner is thinking of when having sex. Some of this stuff comes from other dimensions of who we have been or are to become.
Thoughts are things that travel to wherever they are focused. If we stop thinking of someone else while having sex with a partner we are able to evolve the relationship with a partner to much deeper levels of intimacy because we are not focused outside of the relationship. Looking at the issue of why you are consistently thinking of someone else while having sex with your partner would be appropriate.
What is missing in the relationship? Usually what is missing is the Soul's intention for the relationship and purpose this lifetime. When we mate our personality with our Soul then we are then free to be the mate of each others being rather than just each others personality. We then bring a wholeness to the relationships we engage in.
We create soul to soul relationship in the physical reality as we integrate our Soul's blueprint with our physical being. A main reason why we may be thinking of another person while having sex with someone else is because our blueprint doesn't match the partner we are having sex with. We are then in the same physical space as someone but living in two different worlds.
With a soul to soul relationship we become complements on more than a physical level. We create a trinity within us and consciously feed our relationships with more than our body parts and senses.
We begin to truly serve each other as well as the rest of the beings in the world. We cease thinking of someone else while having sex with a partner because our foundation for who we are becoming in our world will also be complementary to our sexual partner instead of enmeshed with our partner.
Then the sexual experience becomes a vehicle for God and Goddess as "One" to shower the earth with cosmic light substance like trillions of stars transforming this reality.