The Civil Partnership Act 2004 provided for same sex couples to form civil partnerships, which then grew into The Marriage (Same Sex Couples) Act 2013 so that same sex couples have the option of getting married or forming a civil partnership. It is also now possible to register a civil partnership as a marriage fairly simply.

The option of civil partnership or marriage hasn't yet been extended to opposite sex couples. Everyone can chose a civil ceremony or a religious ceremony, everyone can get married but only same sex couples can form civil partnerships, and this is what The Civil Partnership, Marriages and Deaths (Registrations etc) Act 2019 will amend. From 31st December opposite sex couples will be able to form a civil partnership. 

What difference does it make to the arrangements for the children, finances and day to day living of the couple? Absolutely none. The point is, the choice was restricted, and it shouldn't be.