Is sex an effective therapy for back pain?

Can be, yes. Multiple muscle are used during sex which helps keep up mobility. Patients with back pain tend to feel "stiff all over." the body releases pain relieving chemicals during sex which also helps. One key component is to find a very comfortable position (try being on top) which is especially important during orgasm.
Not necessarily. Sexual activity may be relaxing. Orgasms release neurotransmitters and hormones like endorphines, dopamine, oxytocin, relaxine some act as opioids which help to control pain. Obviously the effect is temporal. Depending how people use their back during sexual activity the pain may worsen.