The issue of remaining healthy and free from any unwarranted diseases is natural for every person. It ultimately means that it is totally justified to make sure that you don't contact any such diseases. This is even truer, in today's time when the spread of sexually transmitted diseases has reached epidemic portion.
Getting married has its own risks on the ground of such sexually transmitted diseases as you won't have any idea about the history of your spouse, if it is an arranged one. To ask for a medical test doesn't necessarily means that your partner's behavior or personality is being questioned.
Diseases are also contracted due to transfusion of contaminated blood and sharing of needle as well. Then there are genetically transmitted diseases and others like blood group compatibility.
Blood group compatibility may not be of much significant proportion unless the Rh factors of the partner's do not match. This may prove harmful for the couple's second child. Having prior knowledge is very much helpful to avoid any difficulties in later life which will have direct consequences in your life. There is another important test which you must be aware of, and that would be that of Thalassemia trait. If one of the partners is Thalassemia minor and other healthy, the child is likely to be healthy. However if both the parents are Thalassemia minor, the child could be in a life threatening situation.
However, the most important and serious diseases that you might contract as a result of your partner is HIV or Hepatitis B. The former disease has a huge stigma around it.
The issue of carrying out pre marital test is not about trusting your respective partners. The tests are just precaution to avoid serious ramification. In fact, today's generation believes the issue of pre marital medical tests is completely rational and will definitely assure a sound start to a healthy relationship.