One part disability, two parts emotional nightmare. If you ever have a conversation about infertility with someone who is experiencing it, it is essential that you understand this.
Couples with infertility problems deal primarily with three different types of boors. The first is the tactless boor, who immediately bombards the couple or individual with stories about how they or someone they knew tried for years to have children with no success, and then a little while after they stopped trying, BOOM! they had three kids, one after the other, so you see, all you need to do is to stop trying so hard, there's nothing else to it! Yes, thank you, glad your medical advice is so sophisticated and insightful, but how do you know you have the least clue what you're talking about? Every infertility case is different, and the causes are hardly constant.
Yes, infertility is sometimes related to stress or nutrition and can be corrected with nothing more than a few lifestyle changes. But other times it's physiological, and the possible causes (especially for the female) are almost countless: the ovaries, the eggs, the fallopian tubes, the cervix, the uterine lining, her hormones, and so on. Some of these malfunctions can self-correct. Most do not. But the worst part is that even the doctors won't necessarily be able to tell what the actual problem or problems are, even if they happen to see the pregnancy fail right in their office. There's just too many variables.
The second is the Fundamentalist boor, who maintains that if the man/woman/couple can't have their own children, it must be God's will and it is immoral and presumptuous to try and correct it.
The simple problem with this position is that infertility is not caused by failing to "hit the jackpot" month after month. It's a medical problem, and in that sense it's no different than seeking treatment for a cancer or an artificial limb replacement. The fact that it doesn't directly impact the lifespan or activity of the would-be parents doesn't make it any less a disability worthy of correction. Only a Christian Scientist can and would take such a position with total integrity--and even then it's still rude to say so.
The third is the "why do you want children at all?" boor. Some people don't like small children, or they think that they all grow up to be ingrates, or they think that overpopulation is the worst problem the world is facing today, or they just like making jokes without thinking about them first. Regardless, what they're basically thinking is that the best way to reassure you about not being able to have children is to try and convince you you really don't want them in the first place.
Now, it's bad form to tell your friends what it is they do and don't want in any case, but when it comes to infertility this is particularly painful. Some people, both men and women, only realize they want children after they reach a certain point in their lives; others spend their entire lives wanting to raise a family of their own. When someone tells you to your face that your lifelong ambition is a foolish waste of time and money, it's hard to take it kindly.
Be kind, and understanding, and supportive of the couple's efforts. That's all, really, and it's good advice for any situation where someone is pursuing medical treatment. They don't want to hear "it's all for the best" or anything like that. If you're going to try and make them feel better, try and make them feel better. Don't presume you know what's best in the grand scheme of things. The odds are rather strongly against it.