Why do Koreans pronounce 보제 (菩提) as 보리?

 ANSWER: Because 보제 sounds too similar to 보지, which is considered to be a vulgar reference to the female sexual organ (vulva).

The Chinese characters 菩提 (보제) are used to represent the sound of the Sanskrit word "Bodhi," which means "awakened" or "enlightenment." Buddhists use the word to refer to spiritual enlightenment. The Chinese pronounce the characters similar to "boti" (보디). Koreans would normally pronounce the characters as 보제, but that would create a problem for Korean Buddhist monks reciting Buddhist scripture because 보제 sounds too similar to 보지, a vulgar reference to the female sexual organ. Koreans, therefore, changed the pronunciation of the word from 보제 to 보리 to avoid the problem. They also changed the pronunciation of other 보제 (菩提)-related words, including 보제수 (菩提樹) and 보제심 (菩提心), which they changed to 보리수 and 보리심, respectively. 

They also changed 보제문 (菩提門) to 보리문, which should be translated as "The Gateway to Enlightenment," not as "The Gateway to Excitement."

From Dong-A's Prime Korean-English Dictionary