You can always get crushes on married people. It does not mean that just because they are already taken, they are already less attractive. The important thing is that you will let it stay that way, just a crush. You know that it will be difficult to keep your feelings from time to time but as long as it is just a crush, you will be okay.
You may admire the way that the married person handles his/her relationship. Your wish may be that your future partner will be as understanding as the person that you admire. Some say that married people who start to become attracted to others may not be having as much fun in the relationship as before. This is definitely something to look into.