Arguably monogamous relationships do not need to favour sperm competition, so there is less natural selection in sperm count or swimming abilities.
With species where there is sharing of females for procreation it is the male who produces the biggest abundance of marathon-swimming sperm that statistically will have the best chance of fatherhood. It literally being a first to fertilise race, so speed, quantity and distance are crucial factors. Natural selection would therefore favour that when promiscuous behaviour prevails.
The article mentions alcohol use, smoking and egg quality. I am sure smoking and drinking have reduced substantially over the recent few generations, rather than increased. Egg quality maybe declining as women wait later in life to start having children.
