Even if she's reporting directly to the CEO now, that raises its own questions, in particular for other employees. If I'm at her same level, do I feel like I'm going to get a fair shake if she and I are gunning for the same promotion? Down the road, as the organization grows, is she forever tied to reporting to the CEO? How do you ensure there's never a quid pro quo exchange that opens you to a lawsuit?
It's those kinds of questions that scare most organizations away from having direct reports romantically involved. It's especially fraught in a relatively unstructured organization like a startup where things like promotions and org changes can be fairly sudden and subjective.
Even if the reality is that there's no favoritism, it opens you up to lawsuits both from the people who are involved, and from their coworkers. If they break up, and then he demotes her or lets her go, lawsuit city.
I've seen this firsthand at a major venture startup (but wasn't involved in it myself) - a fiancee/employee decides to leave an officer for one of the other officers of the company. Disastrous for the company.
Anyway, it's their company, and I respect that they can do whatever they want - I only raised this issue because it happens fairly often in the startup world that a very tiny company wants to hire someone in a relationship with an existing employee, and there's a lot of issues you bring on board when you do that.
It would be different if we were inserting Michelle into some rigid organization structure, especially if she was reporting to somebody other than our CEO. We don't have that kind of structure, though, and even if we did, we would never tell a frontline manager to hire somebody based on nepotism. That's a sure-fire way to kill a company in my book.