"But see this study where straight men say they're having sex with men in rural areas and still say they're straight!"

This might shock you, but bi men are notorious for lying about their sexual orientation to any social science researcher, be it this or the "prison gay" phenomenon. Many of them, privately, identify themselves as bisexual and will disclose this to people they trust. They will obfuscate the data of people they don't trust to keep themselves safe in case they get caught out because of how bi men are looked at by the straight people they are surrounded by.