Are the thoughts and desires that arise during prayer due to interacting with and meeting women considered polytheism, especially given that this sometimes leads to losing the prayer?

What the questioner is experiencing is not polytheism (shirk), but rather sins and major transgressions. Among them is the laziness in performing ghusl (ritual bath) from janabah (state of ritual impurity after sexual activity) until the prayer time expires, especially since the cause of janabah is transgressing the limits of Allah by touching or associating with a woman who is unlawful for him.

He must fear Allah and cease committing sins. Relationships with foreign (non-mahram) women are forbidden and necessitate immediate repentance. What he finds in his prayer is due to his actions and his following the footsteps of Satan. Therefore, he must rectify his actions to achieve humility (khushu') in his prayer.