I was wondering the same thing and found this on wikipedia, it is about woman but I'd bet its the same for men:
"In addition to its central role in reproduction, some studies have made claims that semen may have certain beneficial effects on human health:
Antidepressant: One study suggested that vaginal absorption of semen could act as an antidepressant; the study compared two groups of women, one of which used condoms and the other did not.[15][16]
Cancer prevention: Studies suggested that seminal plasma might reduce breast cancer by "not less than 50 percent."[17][18] This effect is attributed to its glycoprotein and selenium content, with apoptosis being induced by TGF-Beta. A related urban legend parodied these findings and claimed that performing fellatio at least three times a week reduced the risk of breast cancer.[19]
Preeclampsia prevention: It has been hypothesized that substances in semen condition a mother's immune system to accept the "foreign" proteins found in sperm as well as the resulting fetus and placenta, keeping blood pressure low and thereby reducing the risk of preeclampsia. A study shows that oral sex and swallowing semen may help make a woman's pregnancy safer and more successful, because she is absorbing her partner's antigens.[20]
Increased libido: Another hypothesis has emerged that absorption of the testosterone contained in semen through a woman's vaginal walls during sexual intercourse (or even through the act of swallowing semen) may increase her sex drive."