Draco is simultaneously communicating two emotional states. Willingness (with his words, which are 100% consistent with consent), and discomfort (with a good bit of his body language.)

Harry is fully aware of Draco's body language, but he's prioritizing Draco's spoken words, largely out of respect. "I don't believe the words you are clearly saying directly to my face" is not a very respectful position, after all.

When the words stop, and all that's left is body language, Harry will reevaluate if necessary. Draco is adamant that Harry will see no reason to do so. We'll find out next week, I guess!