Is ghusl necessary due to madhy that flows merely upon seeing women in regular movies or on the street, and is it necessary to change the state that caused its flow in order to pray?

It is obligatory to avoid looking at women, whether in movies or on the street. If one's gaze falls upon a woman, it must be averted immediately, in compliance with Allah Almighty’s command: "Tell the believing men to reduce [some] of their vision and guard their private parts."

As for madhy (pre-seminal fluid), when it is discharged, one must wash the part of the body it has touched and perform ablution for prayer. A major ritual bath (ghusl (definition)) is not required. It suffices to sprinkle water on the clothes affected by it. There is no need to change one’s posture from what it was when the madhy was discharged, unless you were in a forbidden state, such as watching something impermissible, in which case you must abandon that state.