Yes many are obese, and it can definitely be a struggle, but that doesn’t make being healthy an unrealistic expectation. It’s highly realistic, and many people are healthy.

Only if you think humans are slaves to instinct and are defined by them.

Man also instinctively eats lots of sugars and fats because they are high in energy, so is restraining oneself to a healthy lifestyle unrealistic?

It just depends on your definition of failure. Did the marriage fail to make people happy? Not necessarily. Did the marriage fail in its stated aim to bind two people forever? Yes definitely.

I personally think a divorce is usually a failed marriage (unless the marriage was specifically intended to be limited time) but I don’t think that failure is always a bad thing.