My understanding is that marbling is intramuscular fat and it is not at all unhealthy. It's not the same thing as that thicker glob of fat you see in a rib eye or prime rib or edge of another cut that hasn't been trimmed that well.
I found my source for this, which says:
The fatty acid profile of beef is unique in that it varies based on where the fat is actually located. For example, the external fat on a cut of beef is mostly saturated fat. However, most of the external fat is removed during carcass fabrication at the facility, trimming at the grocery store or by the consumer during meal preparation, so very little external fat is generally consumed. On the other hand, intramuscular fat, or marbling, is rich in monounsaturated fats. In fact, monounsaturated fats, which are the same kind of fat found in avocados and olive oil, make up more than 50% of the fatty acid profile of intramuscular fat.
Now, my source is a document I got when I was a sponsored runner for "Team Beef". So it's not an unbiased source, but I think there is some truth to it. I'm not sure if the less than 50% that is not monounsaturated is enough to be unhealthy.