This emobodies two forms of discrimination; gender bias, and bias against one form of Christianity (Catholicism) for another (Protestantism). It has been an embarrassment for quite a while for the United Kingdom that this inequality exists at the very top of the institution.
The government’s plans to scrap this discriminatory law were abandoned after opposition from the Church of England . The Church of England has vested interested in boths forms of discrimination as embodied by the Act of Settlement of 1701, as (especially in history) it embraces a theological patriarchialism whereby males are set above females, and, has benefitted greatly from State support as a result of historical battles between Protestant Christians and Catholic Christians. Hence why the Church of England has fought against this.
The UK's second chamber of government is called the House of Lords. 26 Anglican Bishops and Archbishops exercise an anachronistic right to sit in this legislative body where they are known as the Lords Spiritual. These particular Bishops have in recent generations played a persistent, active, aggressive and decisive role in battling against anti-discrimination measures against gays in the present time, and against blacks, slaves and women, in older times. This is where much of the effective opposition to government plans to remove the bias against Catholics and women comes from.
So, the Queen herself accepted its effective abolishment at the Commonwealth level, above the government.
"The leaders of the 16 Commonwealth countries where the Queen is head of state unanimously approved the changes at a summit in Perth, Australia.
On scrapping the ban on future monarchs marrying Roman Catholics, Mr Cameron said: "Let me be clear, the monarch must be in communion with the Church of England because he or she is the head of that Church. But it is simply wrong they should be denied the chance to marry a Catholic if they wish to do so. After all, they are already quite free to marry someone of any other faith."
Scotland's First Minister Alex Salmond welcomed the lifting of the ban but said it was "deeply disappointing" that Roman Catholics were still unable to ascend to the throne. [...]
The royal author Robert Hardman said there had been 11 attempts in recent years by individual MPs and peers to change the succession laws.
The laws are not a matter for the 54-nation Commonwealth as a whole, only for the 16 countries which have the Queen as their head of state, known as realms.
These are Britain, Australia, New Zealand, Canada, Jamaica, Antigua and Barbuda, Belize, Papua New Guinea, St Christopher and Nevis, St Vincent and the Grenadines, Tuvalu, Barbados, Grenada, Solomon Islands, St Lucia and the Bahamas.
BBC News (2011 Oct 28)
It seems that even today, Christianity is embroiled in its usual inter-denominational power struggles, and battles still against equality; a problem I look at much more fully in Legislation and Faith: Section 7.1: Bishops in the House of Lords.
 The Telegraph (2011 Apr 25) Alex Salmond calls for clarification on Act of Settlement. http://www.telegraph.co.uk/news/ukn
 BBC News Girls equal in British throne succession (2011 Oct 28).
 Barnett, Hilaire Constitutional & Administrative Law (2004). 5th edition. Originally published 1995 by Cavendish Publishing Ltd, London UK. Page 438.