Paraphrase (reword) the following sentence so that it's easier to understand.

"America aviation was from its very beginnings marred with sexist and racist assumptions."

America from the beginning was full of sexists and racists.

Then maybe explain somewhere what somethings are ...

Sexists are people who judge people based on gender.

Racists are people who judge people based on race or color of skin.

Americans have always been racist and sexist in their beliefs.