Is vigorous, high intensity exercise or prolonged, intense sexual activity safe if you're prehypertensive but otherwise healthy?

Start slowly: If you haven't been very active prior, ease yourself into exercise. High-intensity interval training can be great but also predispose you to injury if you don't have some fitness base. Otherwise, if you have no other issue, prehypertension isn't particularly concerning and exercise, including sexual activity, is good for you. Listen to your body though, and consider touching base with your doctor