You may find it here in full length, or read the first few paragraphs below:
Dear boy, do not be at all ashamed that you desire a girl, or that a girl longs for a boy, but see to it that it leads to marriage, and not to fornication. Then there is nothing disgraceful about it — as little as eating and drinking is a disgrace. Celibacy is supposed to be a virtue, but it is a veritable miracle of God, just as if a person did not eat or drink. It is beyond the capacity of a healthy body, not to mention the incapability of sinful and depraved human nature.Source: Luther's Works (American Edition) v52, p273.
There are not many virgins to whom God granted a long life; rather hurriedly He whisked them out of this world, like Cecilia, Agnes, Lucia, Agatha, and others like them.
I know full well how noble that treasure is, but also how difficult it is to preserve for any length of time. If in every town, there were five boys and five girls, all twenty years of age, completely pure, with no experience of natural discharge, then I would be right in saying that the state of Christianity was better than in the days of the Apostles and martyrs.