On the contrary, sperm competition occurs frequently in many animal species (and more often than you'd think in humans - the stats are quite enlightening). Since this trait increases the chances that your sperm will win and pass on your genes, it is a very useful evolutionary trait.
The length of this
competitive “window” might be as short as 2
or 3 days (Gomendio & Rolda´n, 1993) or as
long as 7 to 9 days (Smith, 1984). Using an
intermediate estimate of 5 days, Baker and Bellis
(1995) argued that the questionnaire data
they collected on female sexual behavior indicated
that 17.5% of British women “double
mated” at some point during the first 50 copulations
in their lifetimes.
