It's simple biological design.
Female mammals only have sex when in season, for dogs that's a weeks window 2x a year, human females are in season every month for 2-3days.
If a male attempts sex out of season or with a pregnant female it will usually get attacked/bitten.
In the words of Bloodhound gang, "you and me, baby, ain't nothing but mammals, so let's do it like they do on the Discovery channel"