Well, I'd assume we'd get divorced, I'd stop being on his insurance, he'd stop getting tax breaks. Our finances wouldn't actually ever join, it's not like we'd be living together, forming join bank accounts, etc. Basically nothing more than a legal arrangement.
Let me just lay down a little truth for you about the tax benefits of marriage:
If you file two separate returns, there are none. You're basically just like two unmarried individuals filing their taxes.
The only way you get any tax benefit is that, if filing jointly, you combine your incomes, exemptions, etc. and your tax bracket cutoffs. Effectively, this means that if both parties are working and make similar amounts, there is no tax benefit. There may even be a tax penalty -- many people in this situation do file separately.
The only way you get a tax benefit is if one party is making a lot more money than the other, like, say, a single-income household living together. In this situation, the exemptions and bracket cutoffs of the second party, who is making less income, are used to reduce the first party's tax liability. Simply put, the primary purpose of the system is to allow one person to work, one person to stay home being a homemaker, and still get the tax benefits as if they were both working.
So, firstly, there would be some financial combination. Otherwise there's no tax benefit. One of you is going to be filing returns for both of you, and they're going to need your W-2s, etc., to do that. Then you guys will also be paying your bill, or getting your refund, together, and you'll have to figure out how to divide that up fairly. (Keep in mind that your respective tax withholdings will also be combined, and they're unlikely to be equal amounts.) And unless there's a great disparity in income between you two, there will be no real savings at all. This leaves aside all of the other issues and concerns with being married -- this is just specifically about one of the primary benefits you're trying to get. You will not get it the way you're envisioning this setup.
The reason people don't do this is not because it's fraud (how would an insurance company prove you got married for benefits? most people who would do this aren't dumb enough to post proof of their intention to defraud the insurance company on the internet), it's because it doesn't actually work well and you probably won't get the benefits you're expecting.
