My site is about sports and I carry a banner for a sports betting site. would this be allowed?
Are you only running the adverts because you make income from the gambling banners? Do you make income in any other way from the site? If the gambling ads are the only income it will look, no matter if it is the case or not, that you are bending the rules.
However, it would take someone to report it before Google even suspected anything if you are advertising on sports terms and not mentioning gambling.