Sexual attraction is strongest at the beginning of a relationship. Nature's goal is to produce children, and historically, people usually had a child after nine months, before contraceptive pills were invented.
If the meaning with your relationships is to be as much as possible infatuated, it is optimal to change partners every time the passion decreases. But how rewarding is such a life in the long run?
People think about many different things before they reach orgasm. It is not significant, it's just a way to climb over the wall and reach orgasm.
You say that you may be afraid of "commitment". Think about what you really want in your life. Is wild sex your only goal? More.
I have talked to several men, and they said that they dream of sleeping with many women, but they also understand that their wives would not appreciate it. Fantasizing about other women, helps them to calm down their instincts, which otherwise would push them to have sex with other women; In that way they can stay with the woman they love.
Men are unfaithful, watch pornographic material and dream of other women. Natural selection has favoured men who want to have sex with as many women as possible. However, men also want children, family, lasting love, feeling of solidarity, emotional security and to give love and affection. Priorities may change with age. Only you can decide what is important for you. Sometimes our needs can collide. Can you combine your needs or must you prioritise?