Why does it burn when my boyfriends semen enters my vagina?

Multiple reasons: The most common reason is that the vagina is slightly acidic and the semen is alkaline making for an acid/base reaction with "burning". However, the other reasons include an ongoing infection by one or both parties as well as vulvar or vaginal tissue injury. A GYN MD can check you for infection during an examination and if found your boyfriend may need to see his MD.