What does it mean if my wife breaks out in her private areas almost every time we have sex?

Allergy or follicles. Is there an allergic dermatitis breakout or a folliculities breakout from bacterial infection of the hair follicles from friction and heat... This would have to be evaluated by seeing the breakout and defining what type of skin eruption it is.
Pimples or swelling. If you mean swelling she has a friction induced hives. This can be controlled by taking a over the counter antihistamine a half hour prior. Zyrtec or Claritin (loratadine) can last for hours and are safe for almost anyone.