1 Answer

You apparently have spontaneous hematospermia, i.e. blood in semen. It happens to many men from time to time and generally is not a sign of STD or anything serious. Some cases, but not all, follow trauma to the perineum (the space between the rectum and scrotum), e.g. after a straddle injury, or painlessly after a bike ride, off-trail motorcycling, horseback riding, etc. Over the next few ejaculations, the color will shift to brown then fade.
See a health care provider (e.g., a urologist) if bright red blood persists more than 2-3 ejaculations or if there still is brown discoloration after several more, or if the problem comes back repeatedly. Of course, if you have other STD symptoms (discharge from the penis, painful urination, etc) don't wait--get it checked out right away.

Source:http://www.medhelp.org/posts/STDs/Discolored-semen/show/245544