Get a yeast infection almost every time i have unprotected sex with boyfriend, 1st partner; he doesn't have yeast infection. what might be the problem?

Not boyfriend: The problem of yest infections is not usually related to a partner...it's not an STD (sexually transmited disease) but rather an overgrowth of your own flora (yeast re everywhere). Have you taken any antibiotics lately ? (a common cause for yeast infections). It is important to ask if you are diabetic or have/know of any other medical conditions which might be related.

Easy to test...: Use condoms every time for a month and see what happens. Then, don't use condoms for a month and see what happens (use another form of birth control; otherwise a baby will come). If there aren't any vaginal symptoms when using condoms, but one gets "yeast infections" when going condomless, then the problem may be the semen. Semen in the vagina may be decaying and become food for yeast or bacteria.