As long as your topic is not too saturated (and I wouldn't think that topic would be), you are the one that ultimately decides how successful your site will be. Creativity plays a large role and as attackdepression said, you have to be willing to think outside the box and do something different than other marriage sites out there.
The biggest mistake I see people make online is that they see a successful site concept and build a site very similar, but they don't do anything to make their sites stand out. MySpace is a great example. Ever since MySpace has become big, thousands of copycat sites have sprung up. Notice you don't hear much about them because MySpace already has a large share of the market and these copycat sites don't offer anything new to steal much of their market share.
If marriage is your topic, spend some time researching other marriage sites out there. Also, creating a site that has an emotional attachment helps too. In other words if you've experienced what you're writing about, you are more likely to engage yourself in the site and you'll spend more time coming up with creative, helpful ideas that will stick.
For example, someone who has never gone through a divorce may not build as useful of a website on divorce as someone who has actually experienced it. If you've experienced it, you know how your target audience feels and you are capable of creating a site that will appeal to their emotions. This is
HUGE and often neglected by people who build websites. You have to
CONNECT.
That means you have to know who they are and be willing to help or solve their problems, whatever they may be. You've got to learn to speak their language. That's why it's so important you define who you are targeting before you begin building your site. The better you connect with them, the more credibility you'll build.
Once you've defined who you are targeting (newlyweds, etc.), it will help you develop the right kind of content. Defining your audience is a step often overlooked but it's a very critical step.
Grab a sheet of paper and ask yourself the following questions...
1) What topic interests me the most and how can I narrow it down?
2) Who would I target? (define gender, age, etc, The more specific the better)
3) What would these people want to know?
4) How will I present this information they need?
5) How will I make my site stand out from the other gazillion sites in my niche so I don't get swallowed up in the competition? (HUGE!)
6) How can I use my personal experience with this topic to influence, inspire, and educate my audience? (Another biggie!)
It may sound very elementary to some, but 95% of people who attempt to build content websites leave out at least one of these important steps. Remember you are not building your site for yourself, you're building it for your audience and your goal is to meet their needs and connect with them. Don't forget to really understand what their needs are.
No connection = No money.
People love to know that you have personal experience with what you're writing about. Your goal is to come across as THE expert for whatever topic you choose. That's how you build credibility.
So to answer your question, your destiny is largely in your own hands. Choose the right topic and add in some creativity, passion, etc. and you're on your way!