It could be an underlying medical issue - like one partner's fertility.
Or you could be having sex too often.
Kind of TMI, but the head of a man's penis is designed to actually REMOVE semen from the vagina on withdrawal. Back in our cave days, it helped ensure that the male could remove a rival's semen in place on his own.
Additionally, it takes several days for a man to produce sperm, so if you're having frequent sex, you could be doing yourself a disservice by replacing the higher sperm-count semen from previous encounters with lower sperm-count semen from later sexual encounters.
Stress is also a factor as it affects the hormones your body produces, which in turn affect your fertility.
at 6:39 PM on Jul. 8, 2009