It's a bit different - you can deny service to people; however, if offering or denying service is conditional on consent, then this means that this consent isn't freely given and thus "doesn't count", doesn't give you any rights to handle that data.

It's done in the same manner as with other consumer contracts - there's a broad range of contractual terms that (in EU) automatically are unenforceable if they're put into a "take it or leave it" consumer contract; GDPR clarifies that permission to use private data is one of such terms; this permission cannot be transferred by some term in a nonnegotiable contract.

I.e. if customer A clicks "agree", customer B clicks "disagree", and you deny service to customer B because of that - then this means that the "agreement" of customer A (and everyone else) is worthless to you, it means that these clicks don't indicate freely given consent and thus do not give you permission to use their data, as customer A can reasonably claim that they did not really want you to use that data in this manner and they clicked "agree" only because you'd refuse them service otherwise.

The legal wording is such that you can't (and shouldn't be able to) gain GDPR-consent unless the users actually want you to do the thing you do with their data; GDPR requires that they know what exactly you'll do, and they without any coercion give an explicit opt-in indication that they want you to do it, and they can freely revoke that permission.