unexpsoc
You, as a man, are in no different a position to the female partner of a woman who has given birth. For every child born after 1st April last year, there is a period of maternity leave for the human being who bore the child, and had the physical and mental risks and disadvantages that went with that.
The mother (i.e. the person who bore etc. etc.) can choose to conclude her maternity leave early and switch the rest of the maternity leave into Shared Parental Leave, which has the flexibility to be taken by either parent, all in one go or as a mixture.
Companies are not obliged to have enhanced maternity pay at all. If they do have enhanced maternity pay, they are not obliged to mirror its provisions onto shared parental leave, just as having enhanced maternity pay doesn't preclude having statutory only sick pay, or vice versa.
You are correct that this may continue to disadvantage women in enhanced pay roles as it may be more affordable for the family to have the woman stay off longer. However, if the government had not introduced this distinction, it is clear that many companies would simply have pulled enhanced maternity pay packages as soon as possible.
You may also be on stronger grounds if, for example, your company offers a year full pay for maternity and your wife has taken six months (so you want the second six months to match your firm's second six months) than if your firm pays full pay for six months but your wife has already taken six months of leave. The goal of the policy was not to 'double up' enhancements.
HTH.