If you don't want to get married, you don't have to, and no one can force you to (in this country any way).
To me, marriage is important. I'm not straight, and while I'm fairly young, I'm not so young that my being able to marry the person I love would be granted to me eventually.
The case for marriage to me is fairly simple - it means something. It's not to me about what it means in the eyes of the state or in whatever god you believe in, but in the view of society at large. Marriage is the way of legitimising your relationship. People know what it means, and what it means is important. Civil unions and such are all very well, and they may confer the same rights, but it doesn't confer the same
meaning.
A really bad thing about marriage is the pressure it puts people under. First of all there might be an expectation that if you are in a relationship for a number of years then people might start asking if you are getting married and if not, why not? Then there is the nervousness involved in whether to propose and if so, will you be accepted. Then there is the pressure of the day itself. If marriage didn't exist we would be spared all of this.
It is also arguably rather reckless and naive to assume that you will always feel the same way about the same person for life. But of course there is always the divorce option, but then the fact that divorce exists means that marriage is irrelevant, because it can always be dissolved on request.
I've been with my girlfriend for four years now, and this past Christmas we agreed that we're going to move in together. And that led me to wondering - do we then get married? I want to get married, I know it sounds a bit silly as a rationale, but I want to see my name and hers next to each other on the family tree my Dad's spent the past 15 years making. I didn't know whether she felt the same way about it as I did, I had the vague idea she'd been a bit against it when it had come up before. Also she's six years younger than me, and I didn't know if she was ready for that in her life. So after a long silence together in the dark, and her assuring me that whatever I wanted to ask I should just ask it and she wouldn't take it the wrong way, I asked at what point we should get married. Call it popping the metaquestion. Her answer was when we're living together. Asking her that was incredibly nerve-wracking, but it means that now I don't need to worry about rejection when the time comes. We're not engaged, but I guess you could say we're engaged to be engaged, and moreover we're both on the same page about what the timescale for that is. When it comes to it I don't plan anything big regarding proposal or a wedding. I'm certainly not going to do what my cousins did (one in a Cornish cave, the other in a Lapland ice palace). But I do want my family to see me exchange vows with the person I love, and who I want to spend my life with.
I guess this is all a bit rambly, but the gist is this - marriage means something to society at large, but what's more important is what it means to you. I won't deny that marriage has some pretty shady history - legal spousal rape for one - and for that reason I can understand why some people may not want to participate. That was a big divide in the LGBT community - some wanted marriage rights so they could be like straight people, some didn't want marriage rights because they viewed the entire institution as corrupt. I choose to believe that marriage means something different to each generation, and that modern couples are remaking marriage into something better and fairer than it used to be.