If someone has hpv 16, how do they have sex? and will having sex keeps reinfecting someone? how to prevent cancer risk apart from the hpv vaccine?

HPV16 (or any HPV) is no reason to avoid sex. Nothing bad happens to most exposed people; let partner(s) know and let them decide. If vaccinated, they won't be at risk. People do not catch the same HPV type repeatedly: once exposed and infected, people are immune to new infection. Prevent cancer by following doctor's advice about follow-up. With proper follow-up, nobody gets invasive cancer.