If a civil partnership doesn't include enough rights or security for homosexual couples then those rights should be strengthened and the issue of marriage left alone . Leaving heterosexual couples unaffected in their current status . And not flushing thousands of years of civilisation down the toilet ,thanks to an imported neo liberal fad from a country that hasn't met a civilisation yet that it hasn't tried to obliterate off the face of the earth .
Except equal marriage allows for equal rights better than any improved civil partnership package.
For example: in immigration, where spousal/family visas are an issue - Ireland will accept marriage certificates from any country where same-sex marriage is legal, and any country where same-sex marriage is legal will accept a marriage certificate from Ireland - having a separate civil partnership complicates this.
Plausible deniablity is also a massive issue - i don't think you could accuse anyone in a same-sex marriage of not being out and proud, but sometimes there are advantages to not immediately outing yourself every time you have to show personal ID like a passport, or declare your marital status on a form - its better that it just says "married" not "in a civil partnership". Imagine if you were a gay person who had to visit family in Uganda, or visit Russia for work? - there will be other circumstances where this keeps people safe too, and it also makes it harder for potential employers/landlords/providers of goods and services to discriminate.
Equal marriage should also take away some of the difficulties around marriage that trans* and intersex people face (although this depends on how the law is written).
The Catholic Church, and other powerful religious bodies are not going to face any legal challenges on this. But saying no to equal marriage also denies the freedom of smaller denominations and religious bodies who want to provide marriage to same-sex couples.
I mean, i'm an anarcho-communist and feminist, me and my girlfriend have no interest in getting married, and I would prefer that marriage as a whole was abolished (along with the state, church, and the entire capitalist system), but that's not going to happen soon, so in the mean time, equal marriage is at least an improvement.