I'm gonna say it's keeping the system flushed. I saw a report on BPH (enlarged prostate), and they talked about how much higher an occurance of prostate problems were with men who lived a celebate lifestyle. They were actually having to go in and have their prostate "milked" by a doctor. Not a fun experience from what they say. Similar to having your prostate checked (bend over!), but then they applied a rather uncomfortable amount of pressure to milk out the fluid.
Glad I have a healthy marriage!
