According to the Collins English Dictionary and the Oxford Dictionary Online, to 'marry' someone means to join in marriage with them, uniting closely and harmoniously. Neither definition specifically says you have to marry someone of the opposite sex. In fact, the Collins Dictionary goes further: in the origin of the word, it suggests that it is from the Latin maritus, meaning married, and is 'perhaps from mas male'. Women just don't come into it.
I have some sympathy with Christians who say that the Bible forbids homosexuality and that therefore gay marriage shouldn't be allowed. Some, but not a lot. They've been reading their bibles pretty selectively. Bacon sarnie? Don't mind if I do - but Leviticus 11:7-8 has forbidden it, so sorry, you can't. Tattoo with the name of your loved one? Leviticus 19:28 rather suggests not. Beard trimmer for Christmas? You can't use it according to Leviticus 19:27. I could go on, but you might be rather depressed. You might have to chuck out that favourite embroidered sweater... In other words, using the bible as your defence is pretty weak when you really think about it.
But I'm less sympathetic still with those who say, 'It's icky' or words to that effect. Do you spend much time thinking about what other couples get up to in the privacy of their bedrooms, whether they be heterosexual or homosexual? Because if so, you're the one who is a bit icky. Especially if you feel that your imagination is what makes what they're doing 'wrong'.
And as for those who think that everybody is equal but that marriage is still just for a man and a woman (and yes, I'm looking at you, here, Tony Baldry) well, sez who? Because if you're going to tell me that's what the Church says, the Church has changed its mind on quite a few things over the centuries. Really you're trying to justify your prejudices, ones that you've grown up with and that have no real basis in fact.
So yes, it might be hard to take, but we managed to have women vicars giving communion wafers to divorcees and the world hasn't blown up yet. Give it a chance. Or more accurately, give love a chance. If two people love each other enough to want to spend a fortune telling the world how much they love each other and they want to do that in a particular building, does it really matter if both or neither of them has dangly bits tucked in their trousers?