<<My understanding is that to convert an IRA to a roth, you must make less than $100,000 as either a single or a married couple. (It seems unfair to the couple since the income level is raised for a couple to donate.)>>

Your understanding is correct.

<<If this is true, my question is: what happens if a single man and a single woman who qualified to roll
over into a Roth get married this year? Combined, they make more than 100K. It is also my understanding that they cannot file as married-filing single. Or is that just for contributing?>>

You are correct that they cannot file as married-filing single. It would appear that is they got married this year and converted to a Roth IRA, they would be subject to an excess contribution penalty tax. I will let the experts, TMFPixy and KAT, confirm this.

KAT's web site has a wealth of information

http://www.fairmark.com