The CP is a relatively new, mainly legal contract that comes with much less emotional and historical baggage than a marriage.
Not for same sex couples it doesn't. There is a lot of emotional and historical baggage (just not as historical as marriage). It was not that many years ago that homosexuality was illegal in the country. It took a long time for same sex couples to be accepted (a lot still have issues) and a lot of fighting to get any form a legal status for their relationships.
Lets not forget that the only reason we have CPs is because religious groups would not allow same sex marriage because they do not agree with it. CPs are 'marriage-lite' but in the sense that same sex couples were not allowed to have a marriage, so had to 'make do' with a lesser title to appease religion.
It has taken a lot for same sex couples to even come close to the rights heterosexual couples have and they still do not have the same rights - I know one couple who will not 'get married' because they want to have a church wedding and they are not able to.
I have no real objection to heterosexual couples having the option, but the easiest thing would be for the government to pass an Act to recognise existing CPs but to prevent any new CPs being entered into now the reason for the discrepancy has passed. That will keep everyone equal.
But in the event that they are not married, their mother, their father, is still the person who will inherit their estate and make medical decisions in difficult circumstances.
Not necessarily.If the person is sensible and has a Will and a Lasting Power of Attorney (which is always recommended and especially if you are just cohabiting) then you can leave your estate to whomever you like and appoint whomever you like to make your medical decisions for you. It is only those who do not bother to take any legal steps to protect their partner who end up with problems.
how would infidelity be classed as unreasonable behaviour
It is still expected to be the union of one person with another and fidelity is implicit.
Adultery by law is sex between two people of the opposite sex and can only be used as grounds in divorce by the innocent party. For example, a wife cannot divorce her husband on adultery if he had sex with another man (that is not adultery by law), but that would be unreasonable behaviour. Adultery is difficult in the same way that a wife cannot divorce her husband on the grounds of her own adultery.
CPs can't have adultery as a grounds as they were intended to be for same sex couples and the thought was they were more likely to cheat with another person of the same sex and so could not legally commit adultery.