I recently learned that in ducks, the penis and vagina have coevolved: the vagina of the female ducks gets dead-end sacs and clockwise coils; the penis of male ducks become longest and its morphology changed to adapt to the female vagina.  This antagnostic coevolution happened due to the evolutionary arms race between male and female ducks.
This seems counter-intuitive when we think about natural selection. Indeed, if female ducks are forcibly copulated, they are more susceptible to reproducing themselves and sharing their genetic traits.
From , it appears that only 3% of forced copulations results in fertilization. If this is the case, why has the male phallus evolved this way? Indeed, it does not appear to me to be a good evolutionary strategy.
If  and  are correct, I would think that the duck vagina and phallus shouldn't have coevolved that way. What am I missing?
 Brennan, P. L., Prum, R. O., McCracken, K. G., Sorenson, M. D., Wilson, R. E., & Birkhead, T. R. (2007). Coevolution of male and female genital morphology in waterfowl. PLoS one, 2(5), e418.
 Burns, J. T., Cheng, K. M., & McKinney, F. (1980). Forced copulation in captive mallards. I. Fertilization of eggs. The Auk, 97(4), 875-879.