'Gay people have civil partnerships, marriage isn't necessary'
And black people had black washrooms during the racial segregation period in the US. Sure, they were less well kept and ultimately separate from the bathrooms used by the white majority, but black bathrooms still did the same thing right? They could still wash their hands and use the toilet?
Clearly this was not ok, and I think the marriage/civil partnership distinction is exactly the same. You're basically telling gay couples that they aren't worthy of the special status of 'marriage' (which carries prestige and is deeply embedded into the traditions our society) and must settle for something not only different, but worse, effectively excluding an entire minority from one of our country's most prestigious institutions. In both the white bathroom/black bathroom and marriage/CP, a person is still able to undertake the same function (i.e.wash hands, get partnered, respectively) - it is the principle of the two being separate which is the issue. It is implying that one group (funnily enough, the dominant group in both cases) in society is entitled to one thing, while another group is entitled to another (funnily enough, inferior) thing.
'There is no need to change something which has been in place for hundreds of years'
I hate this argument, it's such a cop out and is often not supported by any reasoning. This very same argument could have been applied to slavery and public executions before they were abolished. In fact, even recently with developments in gender equality, one could make the argument that we have always lived in a patriarchy where females have had very limited rights - why should we change a system which has been in place for hundreds of years?
Well guess what, society and its attitudes change, and therefore we need to account for this by altering our laws and institutions to cater for this social evolution. The old way is not necessarily the right way, as I have pointed out with slavery, females being denied the vote, etc, and tradition is a poor excuse for denial of rights. If you value your traditions over the liberties and welfare of society (in this case, a particular area of society), then maybe you should have a good long think about your morality.
'Can straight people get civil partnered though?' as a way of trying to turn the argument is stupid and detracts from the main point. But yes, I agree that civil partnerships should be extended to straight couples, as the informality of a civil partnership is occasionally seen as more desirable than marriage.
Our politics is not guided by religion - something we should be proud of - so why should this extend to marriage? So many non-religious people use religion as a defence for their homophobic views which makes me think that you don't have a valid reason. In fact, I'm yet to come across a good argument for non-religious people to oppose gay marriage and homosexuality as a whole, so please give me one, there has to be at least one, surely?? If your religion is opposed to it I totally get that and most of my arguments don't necessarily apply.. although if your religion is encouraging you to take a negative stance towards fellow human beings based on sexual preferences which they DID NOT CHOOSE, maybe I'm opposed to the relevant passages of your outdated religious texts.
Oh and by the way, 'marriage should be between a man and a woman' without giving a reason why is the most painfully dogmatic and ignorant argument you can make, and I will automatically assume that you have nothing valid to say in your subsequent argument. Why should it be?!
Above all, WHY THE **** DO YOU CARE? Two people of the same sex institutionalising their love in the prestigious form of marriage has absolutely no effect on your life whatsoever, yet it makes not only the couple happy, but an entire group of people in the UK who may begin to feel like the government recognises that, just maybe, LGBTQ+ people are not the downtrodden second class citizens they once were and that laws should be introduced which recognise this equality?
Edit: Oh, and 'marriage should be for raising children' is also an out-dated and painfully conservative argument. Should we stop infertile straight people from getting married? No because they can adopt, surely? Oh wait, so can gay people ...