Beha’alos’cha, Bamidbar 9:7. Lamah nigara, לָמָּה נִגָּרַע לְבִלְתִּי הַקְרִיב אֶת-קָרְבַּן ה' בְּמֹעֲדוֹ בְּתוֹךְ בְּנֵי יִשְׂרָאֵל. Pesach Sheini.

This is a little complicated, but I think it's worth the effort.

The Torah records the complaint of the men who were prevented from bringing the Korban Pesach with the rest of the nation due to their Tum'ah. What kind of question is this? How on earth did they have the right to confront Moshe Rabbeinu with a complaint about the Torah? They knew the halachah of tumah, they knew that a tomei couldn’t bring a Pesach, they knew that according to the dinim that Moshe had taught them, they were not allowed to bring the korbon. This is all evident in their question: we know that the Torah says we are not allowed to bring the korbon. And still they stood up and said it was not fair? This is how the question was phrased by Reb Aryeh Leib W הרוצה בעילום שמו in shiur in June 2006.
UPDATE:  Since I wrote this, the person who asked me this question passed away.  His name was Larry Wilk, and he was a dear friend who later became a talmid.

There are a lot of possible answers, none of which I like. For example:
Lamah=please explain, that it was more in the way of a plaint than a complaint; or,
We were tamei because we were oseik with the Aron of Yosef, so we’re anusim because we did a mitzvah; or,
We were oseik with a meis mitzvah; or,
This korban is central to our identity as Jews.
And so on. None of these pshatim hits the spot. None of these pshatim satisfactorily explain the tone or content of what seems to be a confrontational argument, a complaint, not a plaint.
Thank you to an anonymous writer, who directs us to the Ritva:  

"Ritva in Megilla 25 says that erev Pesach was their 7th day of haza'ah so they could have gone to the mikva and eaten that night. They held that since they were merely mechusar tevila it's muttar to shecht and be zorek on their behalf and eat that night."

I think the answer is this:
They knew that the Korban Pesach, although technically a Korban Yachid, a korban which each individual is personally obligated to bring, has certain dinim of a Korban Tzibbur. For example, as the Gemara and Rashi in Psachim (also Tosfos Sanhedrin 12b d’h shetum’oh) bring from the Gemara in Yoma 51a many times, it’s like a korban tzibur because it is brought "b’kenufiah", en masse. Second, there are many rishonim in Yoma around daf 8, discussing the status of the Pahr Kohen Godol on Yom Kippur, and see the Shagas Aryeh 38 and Rambam III Tmidim Umusofim 18, (and see Mishneh Menachos 96 about the Chavitin of the Kohen Godol, and the Gemara there on 51b with three explanations for why it’s docheh Shabbos,) that say that any korban, not only a korban tzibbur, that has a zman kavu’ah, can apply the din of tum’ah hutrah b’tzibbur. Now, obviously, these Rishonim do not apply this semi-Tzibbur status to a yachid who is tamei erev Pesach. But the point is that the tmei’im asked Moshe, we know there is a din of tum’ah hutrah be’tzibbur. Since korban Pesach comes be’knufiah, it is like a korban tzibbur even though it is brought by yechidim. So, they said, the din of tum’ah hutrah b’tzibbur should apply to every yachid that wants to bring a korban. And certainly according to those rishonim that hold that the Pahr Kohen Godol had a din korban tzibbur, the same should apply here. This explains why they used the expression "lamah nigara mitoch hakahal" and "b’mo’ado:" these are the words that are used to identify the Korbanos that are docheh Shabbas and Tumah.

So Moshe told them they were right. Their kashe was so good, that they were m’chavein to a din that they hadn’t heard from him– like those tana’im that said that if the Torah was forgotten, they could recreate it misvara, and as Asni’el ben Knaz later did. Their taineh was correct, but only on the basis of what they knew. If there were only one time to bring the korbon, they would be allowed to bring it b’tum’ah. But, he told them, there was also an option of Pesach Sheini. Now that there is that alternative date, it no longer is called a korban that is kovu’ah lo zman.

If the tzibbur was tamei, then it is kavu’ah lo zman, because a tzibbur cannot bring on Pesach Sheini. But since a yachid has the alternative, for yechidim it’s not kavu’ah lo zman.

In other words, it is not the inability to be makriv b’tum’oh that results in the alternative; it is the alternative that results in the inability to be makriv.

It’s immodest, but I like my teretz better than any of the other teirutzim I’ve seen.

Anyway, it’s nice that among all the complaints about not having enough good food to eat, people complained once about not being able to do a mitzvah. The same Ribbono Shel Olam that didn’t give them the food they wanted is the One Who limited their options on how and when to do mitzvos.