Uh.. it's definitely not guaranteed that a penis will cause an infection. If this is a regular occurrence for you, talking to a doctor would be a good idea. In fact, there was some research a while back that suggested that having oral sex first reduced the risk of yeast infections from penetrative sex at a later date. The theory was that the woman's body would be exposed to the foreign microbes in a smaller dose which helps train the immune system to deal with it.