Who knows, being devoted to each other romantically might make them less likely to knife each other in the back if the startup looks like it might take off.
Long story short, while it's not hard to imagine a problem, there are plenty of businesses owned by a couple who are romantically involved.
Could it cause problems? Perhaps. But, it's a startup. You need to be prepared to bail with little notice, regardless, for any startup (because more or less by definition they don't offer much security), so I don't think this needs to be a top concern.
