The classic first signs of diabetes are extreme thirst and hunger coupled often with excessive urination and inability to heal. However, if you let it get to that point, those symptoms are just the beginning of your problems: significant damage has likely been done to your arteries and endothelium and that's bad news for your health and your sex life.
Diabetes is a plague in almost every modern industrialized society. In America, for example, studies have shown that 14% of males have diabetes. That is about one out of every seven guys struggling with a major illness that is literally eating their arteries and organs from the inside out. And this does not even take into account all the males, probably an additional 3-6%, depending on age, that have diabetes and do not even know it.
Diabetes is a huge risk factor for erectile dysfunction and one study estimates that as high as 50% of men with diabetes have significant erectile dysfunction.  The study explains that "in addition to the disease's effect on small vessels, it may also affect the cavernous nerve terminals and endothelial cells, resulting in deficiency of neurotransmitters Additionally, in diabetics, corporal smooth muscle relaxation in response to neuronal- and endothelial-derived nitric oxide (NO) is impaired, possibly due to the accumulation of glycosylation products ." 
In other words, diabetes is very hard on erections in almost every way possible, attacking the endothelium, nitric oxide, nerves and neurotransmitters. What is left when those are damaged or depleted after all? Even worse, diabetes is known for resulting in even more severe erectile dysfunction than normal.  Diabetics score worse in almost every major category including erectile function, intercourse satisfaction, sexual desire, overall satisfaction, and psychological impact.
NOTE: Yes, testosterone has miraculous effects on male insulin levels, i.e. it lowers them significantly, a fact I cover in my book Low Testosterone by the Numbers. This effect of this appears to be much more powerful than I realized. I recently spoke to to a worker in an HRT clinic and she said that ALL of their Type II diabetic patients were able to totally eliminate their need for insulin. This is astonishing. Yes, a few had to keep taking Metformin, but nevertheless, this is a remarkable reversal. This clinic increases testosterone via weekly injections ito around 1000 ng/dl.
And it's no wonder: diabetes is brutally hard on the heart and arteries. Diabetics suffer significantly more strokes, heart attacks and high blood pressure than non-diabetics. Much of this has to do with the fact that arteriosclerosis is accelerated with diabetes due to elevated inflammation, oxidation, fibrinogen, arterial plaque and weight around the midsection. Diabetes raises almost every risk factor for cardiovascular disease and, as I often mention, the Heart and the Penis are intimately linked. Even worse, diabetes can be a silent killer: one European study found that about one third of heart attack patients were diabetic and didn't know it. 
Diabetics can get into trouble even with relatively normal LDL levels because their LDL molecules are the smaller, more deadly kind. They also usually have worse HDL and triglyceride levels to go along with it. In other words, the whole lipid profile is skewed negatively.
All of this is generally leads to severe erectile dysfunction. Yes, diabetics can get some relief from PDE5 inhibitors such as Viagra and Cialis according to the studies, but the studies also show that the results are not as long-lasting. And who wants to be dependent on a pill for their sex life?
So the bottom line is to keep your blood sugar and glucose under control and do everything within your power to avoid diabetes in the first place. This is especially important if you have a family history of diabetes.
What's a fella to do? Fear not: I've got a great list of preventative measures in my link on Metabolic Syndrome Solutions. But start now: don't wait until the damage has already been done. In addition, read my link on Natural Ways to Prevent Diabetes and consider the book Reversing Diabetes, an M.D.'s program to dramatically help and sometimes even completely things.
1) Population Health Metrics, 2009, 7:16, "Diabetes prevalence and diagnosis in the US states: analysis of health surveys"
2) Urol Clin North Am, 2005 Nov, 32(4):379-95, "Physiology of penile erection and pathophysiology of erectile dysfunction"
3) Diabetes Care, Apr 2003, 26(4)1093-1099, "Do Impotent Men With Diabetes Have More Severe Erectile Dysfunction and Worse Quality of Life Than the General Population of Impotent Patients?"
4) Eur Heart J, 2004, 25 (21):1861-1862, "The prevalence of abnormal glucose regulation in patients with coronary artery disease across Europe"