So what if two people are married and they find out they cannot have children? Are they called to chastity?

This is a very common question, and cuts right to the heart of moral discussions - what is responsibility?

In Criminal law we talk about mens rea and actus rheus. To be morally culpable, we say that someone must have intended the act.

For example, if I buy a plane ticket for a friend to Mexico (I'm a good friend like that) and they die in a plane crash, am I at fault for it? I'll probably feel guilty, but I doubt anyone would say I murdered my friend.

So if two people are having sex that is (1) open to procreation and (2) loving and caring, then they are fulfilling their part of the bargain.

If there is some problem with the faculty such that it does not work as intended, then they are not at fault. Just as someone with a heart condition can't be blamed for having a heart murmur.

Furthermore, if we believe in God, then we also believe God can work miracles.

A similar argument is - should a couple refrain from sex after menopause?

The answer is the same. The couple must be open to life and not doing anything to frustrate the possiblity of a child.

So in brief, it isn't immoral to have sexual intercourse if a medical issue may prevent conception, but it would be immoral, for example, to intentionally damage the faculty with a vasectomy or frustrate it with a condom.