In recent weeks, State Sens. Daylin Leach (D., Montgomery) and John Eichelberger (R., Blair) entered the fray with attempts to legislate how couples in Pennsylvania ought to live their lives. The senators seem to think their job is to tell people whether they can marry or not. But it is not their job, and, even if it were, they would not be able to do it.

No matter what they do to ban or legalize same-sex marriage, there is no way to reliably enforce such a law. The problem revolves around three questions: What do they mean by sex? Who determines a person's sex? And how is it determined?

One can rely on X and Y chromosomes in determining sex. But there are plenty of people who have one X and one Y chromosome - the genetic combination traditionally considered male - who look like and consider themselves women. They include those with androgen-insensitivity syndrome.

There are also people who have two X chromosomes - the traditionally female combination - but look like men, produce sperm, and can even have children with their wives. These people have what are known as chromosome translocations.

In any case, are we really going to mandate karyotyping - the profiling of people's chromosomes - before marriage ceremonies?

We could instead use gonads to determine sex. However, some women, such as those with Turner's syndrome, have no ovaries or only rudimentary ones. Men with testicular cancer may have their testes removed. Do Leach and Eichelberger want to tell them whether or not they can marry?

Alternatively, we could focus on the way people look and the gender they assume. But gender identity does not always match the number of X's and Y's, the presence of ovaries or testes, or other characteristics.

The point is that biology is not that simple. There are plenty of examples of married couples who would defy any definition Leach and Eichelberger can come up with. Part of the problem lies in the urge to make rules that cannot be followed.

Embryology tells us that humans are not sexually dimorphic - having only two forms. Rather, there are males, females, and a group of people who are in between. The in-between people are normal. They are our neighbors, friends, and colleagues. As children, they go to school with our children.

These people present to the outside world the way they believe best represents who they are. This is their decision, and society should not try to legislate it. And so society also should not legislate whom they marry. That is their decision, too.

Let us ask Leach and Eichelberger to stop worrying about something they cannot control and perhaps start figuring out how to help the poor, get the economy going, and make America the great nation it once was - "the land of the free."

Thomas A. Marino is a professor of anatomy and cell biology at Temple University School of Medicine. He can be contacted at marino@temple.edu.