I keep getting reinfected with bacterial vaginosis after sex with my boyfriend.why?

Try condoms a while: If bacterial vaginosis keeps returning, a woman may be getting something from her partner. Both persons can examined by a doctor at about the same time, to see if there are other contributing factors. A woman might recover better if she keeps semen out of her vagina for a while, to allow her vagina to naturally self-clean and return to normal. That means using condoms for some weeks.

Changes from sex: Sometimes the ejaculate (seminal fluid) can have a altered pH from your vaginal cavity, causing enough change to favor BV infection. First, are you sure its bv? I would seek an obgyn to perform an exam and STI testing to make sure its not something like trichomonas. Other things such as good hygiene, avoiding douching, not using chemicals or lubricants during sex (all can alter the ph) may help.