How can someone get genital herpes even if they have only ever had sex with people who are negative to hsv-2?

Could be HSV1. Hsv1 generally is considered oral herpes but can be transmitted during oral sex. It is more troublesome in the gential area because it is more sensitive there. Most people with oral herpes have an occasional embarrassing cold sore but not anything life-changing - that's not usually the case with gential herpes whether type 1 or 2. Meds can help treat/prevent outbreaks if troublesome.