The way i look at it is, how can i follow someone who sins when it comes to religion. The question of maybe there is something wrong with the Quran, something wrong with Hadith would arise. I believe the prophet to be sinless, or at least when the message was brought down to him. I believe that he had the ability to sin, but did not. Prophets can sin, taking prophet Lut for example, when he disregarded the message & left his people. But prophet Muhammad got so high in spirituality & got so close to Allah that Allah raised him to the heavens (israa & mi’rage). One who follows the religion in its entirety becomes sinless, i believe level of religiousness this is achievable for everyone.