Going on dates with a non mahram man is itself not permitted and can causes many ethical problems but still we can’t say it’s as an adultery as long as both of them had not have any physical relationship.
As for as the philosophy and condition of polygamy in Islam please go to the below link you will find your answer in a complete way.
https://www.al-islam.org/articles/concept-polygamy-and-prophets-marriages-sayyid-muhammad-rizvi