After 4 years of dating, my girlfriend has been battling chronic yeast infections. gyno says its d/t sperm but why my sperm. are there other causes?

Yeast: It actually seems quite unlikely to me as a urologist that your sperm ( or anyone else's for that matter) could cause your partners recurring vaginal candidiasis. It is possible, especially if you're uncircumcised, that you harbor yeast under your foreskin. In that case you may be passing the organism back and forth so to speak. Get checked out by a urologist.