Will spit/saliva entering penis cause any infection?

Theoretically, yes.. Theoretically, this scenario can induce urethritis by bugs from saliva, but in reality, there has been no specifically designed study to address this concern, and the one-way urine flow will flush the bugs, in case accessed into urethra, out of the urethra with no sufficient time & environment to multiply & set foot there to manifest their diseases. Hence, if any, it is very rare.