Yevamos, Chapter 6, Mishnah 5. The Mishnah states: "Kohen hediot lo yisa ailonis, ela im ken yesh lo ishah u'vanim" - an ordinary kohen may not marry an ailonis, a barren woman, unless he already has a wife from whom he can have children, or he already has children from a previous marriage.
Why the Mishnah specifically mentions a kohen:
The Gemara explains that although the Mishnah speaks of an ordinary kohen, the law is not unique to him: an ordinary Jew who is not a kohen likewise may not marry an ailonis unless he has another way of fulfilling the mitzvah of having children, in exactly the same measure. The kohen is mentioned because the opinion of Rabbi Yehudah is about to be brought, and his disagreement applies to a kohen alone; but according to the first Tanna, the law applies to every person.
The dispute between Rabbi Yehudah and the Sages in defining a "zonah":
Rabbi Yehudah: "Af al pi she'yesh lo ishah u'vanim - lo yisa ailonis, she'hi zonah ha'amurah baTorah" - even if the kohen has a wife, and even if he has children, he may not marry an ailonis, because she is the "zonah" that the Torah forbade to a kohen. A kohen may marry only a woman who is capable of bearing children.
"Va'chachamim omrim: ein zonah ela giyores u'meshucheres v'shenivalah beilas znus" - according to the Sages, an ailonis is not considered a zonah at all, and the term zonah applies only to these three:
A convert - a woman who converted, since we assume that she had relations with gentiles before her conversion.
A freed maidservant - a maidservant who was set free, who falls into the same category.
One who had relations of znus - a woman who had forbidden relations, with a man to whom she is forbidden.
The early authorities disagree about which level of prohibition is meant: is it specifically relations with those liable for kares or for death at the hands of the court, or perhaps even relations with those who violate a mere prohibition - those prohibitions that are only a lo taaseh in the Torah - and even prohibitions stemming from a positive commandment. In any case, according to the Sages, and the halachah may follow their view, the term zonah applies only to a woman of this sort.
In summary: this dispute obviously pertains to a kohen alone, and that is why the first Tanna opened with an ordinary kohen; but the view of the first Tanna regarding the mitzvah of having children applies to every single Jew.