Common causes of yellow semen are mix up with urine, ageing, diet and infection. Semen usually turns a more yellow color as a man becomes older. I assume that your fiancé is young (although you have not mentioned his age) so I rule out ageing as a cause of yellow semen. Food especially high in sulfur such as onion, chives, shallots and garlic can cause yellow semen. This also includes medications and multivitamins, which may also cause yellow color. The most common cause in young is infection and the most common infection is epididymitis, which is caused by prostate infection and can be easily cured with a course of antibiotics. There is no relation with family history of prostate cancer. So don’t worry. Ask your fiancé to take a course of antibiotic with his doctor’s advise. Good luck!!