Which prohibition does one commit when viewing pornography?

Question

Excluding wasting one's seed (hotzas zera l'vatelah) and Bittul Torah (wasting one's time by not learning Torah), which Jewish commandment or law (Biblical or Rabbinic) is one breaking when viewing pornography?

When someone chas v'shalom eats not kosher food it's easy to say exactly which issur they are committing. However, in this case, is it so clear cut? Can it easily be pointed to a negative commandment that the person is neglecting?

Note: I realize that viewing pornography is not in the "spirit" of the law. My question, though, pertains to the letter of the law.

Answer 1

You're looking for the verse (Devarim 23:10):

וְנִשְׁמַרְתָּ מִכֹּל דָּבָר רָע
then thou shalt keep thee from every evil thing.‏

The Talmud (AZ 20) interprets this (based on context from the next verses) as meaning "שלא יהרהר אדם ביום ויבוא לידי טומאה בלילה" that a man should not fantasize during the day and thereby come to have a nocturnal emission. The examples in the Talmud there of things not to do are staring at women, even ugly women, even bright women's clothes, and viewing mating animals. (Pornography seems clearly included.)

It seems clear that if something will potentially bring a man to ejaculate during the day that is no better than having it cause it to happen at night unwittingly.

Tosfot (ibid.) comments that this is a bona fide biblical exposition, but the Rambam doesn't quote the verse in his presentation (IB 21). In any event, it's forbidden on whatever level, and here's something to point to.

Answer 2

From the Shulchan aruch bellow regarding erotic books (even only text no photos) seems 3 things

1. מוֹשַׁב לֵצִים (תְּהִלִּים א, א),
company of scorners tehillim 1.1.
2. וְעוֹבֵר מִשּׁוּם אַל תִּפְנוּ אֶל הָאֱלִילִם (וַיִּקְרָא יט, ד).
You shall not turn to the worthless idols vayikra 19.4 (see rashi).
3. מְגָרֶה יֵצֶר הָרַע.
incites the bad inclination (source below)

And since it is in the Shulchan aruch also (many it is part of #3 above)

4.  לֹא תָסוּר מִן הַדָּבָר אֲשֶׁר יַגִּידוּ לְךָ יָמִין וּשְׂמֹאל

 you shall not divert from the word they tell you, either right or left devorim 17.11

Orach haim 307.16

מְלִיצוֹת וּמְשָׁלִים שֶׁל שִׂיחַת חֻלִּין וְדִבְרֵי חֵשֶׁק, כְּגוֹן סֵפֶר עִמָּנוּאֵל, וְכֵן סִפְרֵי מִלְחָמוֹת, אָסוּר לִקְרוֹת בָּהֶם בְּשַׁבָּת; וְאַף בַּחֹל אָסוּר מִשּׁוּם מוֹשַׁב לֵצִים (תְּהִלִּים א, א), וְעוֹבֵר מִשּׁוּם אַל תִּפְנוּ אֶל הָאֱלִילִם (וַיִּקְרָא יט, ד) לֹא תְּפַנּוּ אֵל מִדַּעְתְּכֶם; וּבְדִבְרֵי חֵשֶׁק, אִכָּא תּוּ מִשּׁוּם מְגָרֶה יֵצֶר הָרַע; וּמִי שֶׁחִבְּרָן וּמִי שֶׁהֶעְתִּיקָן, וְאֵין צָרִיךְ לוֹמַר הַמַּדְפִּיסָן, מַחֲטִיאִים אֶת הָרַבִּים. 

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as @mevaqesh says below this #4 is only if this law existed in the time of the Sanhedrin, but if the shulchan aruch brought from later it is only
5. אחרי רבים להטות.
follow many to pervertShemois 23.2

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It is very interesting that all the above prohibitions (might) apply to women (even number 3 (as is hinted by the law of if a woman can drink wine))

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Niddah 13b

Rab stated: 'A man who wilfully causes erection should be placed  under the ban'. But why did he not say, 'This is forbidden'? Because the man  merely incites his evil inclination against himself. R. Ammi, however, stated: He is called a renegade, because such is the art of the evil inclination: To-day it incites man to do one wrong thing, and to-morrow  it incites him to worship idols and he proceeds to worship them.

There are others who read: R. Ammi stated, He who excites himself by lustful thoughts will not be allowed to enter the division of the Holy One, blessed be He. For here it is written, Was evil in the sight of the Lord,  and elsewhere it is written, For Thou art not a God that hath pleasure in wickedness; evil shall not sojourn with Thee.

Answer 3

I'd add two rather obvious ones:

"You shall be holy" (Vayikra 19:2) (and "You shall be holy to your G-d," [Bamidbar 15:40 et passim], and " You shall be to Me a kingdom of priests and a holy nation” [Parashas Yisro])

and

"You shall not explore after your heart and after your eyes after which you stray" (Bamidbar 15:39)

(Please see the links for discussions of these verses which go a long way toward connecting them to the question.)

Answer 4

Masechet Nidda 13b: to cause an erection is prohibited. (cited in the answer of @hazoriz)

אמר רב המקשה עצמו לדעת יהא בנדוי ולימא אסור דקמגרי יצה''ר אנפשיה ורבי אמי אמר נקרא עבריין שכך אומנתו של יצר הרע היום אומר לו עשה כך ולמחר אומר לו עשה כך ולמחר אומר לו לך עבוד עבודת כוכבים והולך ועובד איכא דאמרי אמר רבי אמי כל המביא עצמו לידי הרהור אין מכניסין אותו במחיצתו של הקב''ה כתיב הכא {בראשית לח-י} וירע בעיני ה' וכתיב התם {תהילים ה-ה} כי לא אל חפץ רשע אתה לא יגורך רע ‏
Rab stated: 'A man who willfully causes erection should be placed under the ban' But why did he not say, 'This is forbidden'? Because the man merely incites his evil inclination against himself. R` Ammi, however, stated: He is called a renegade, because such is the art of the evil inclination: To-day it incites man to do one wrong thing, and to-morrow it incites him to worship idols and he proceeds to worship them. There are others who read: R` Ammi stated, He who excites himself by lustful thoughts will not be allowed to enter the division of the Holy One, blessed be He. For here it is written, Was evil in the sight of the Lord,(9) and elsewhere it is written, For Thou art not a God that hath pleasure in wickedness; evil shall not sojourn with Thee.

Ohel Moed

So that he will not become to wast his semen. To wast semen leads to death punishment, as said in Tora about Er and Onan, and he (G_d) killed him as well. On this case Chazal said (Gemara Yuma 29a) guilty thinking are more serious than the sin itself. Because that one who has sexual intercourse with a single women violate a 'Lav' only and this (who imagine an intercourse) is punishable by death.

A second Gemara, is reported in the answer of @DoublAA, the Gemara AZ 20a&b.

Rambam Issure Bia 21, 19; SA EH 23, 3

וכן אסור לאדם שיקשה עצמו לדעת או יביא עצמו לידי הרהור. אלא אם יבא לו הרהור יסיע לבו מדברי הבאי (והשחתה) לדברי תורה. שהיא אילת אהבים ויעלת חן. לפיכך אסור לאדם לישן על ערפו ופניו למעלה עד שיטה מעט כדי שלא יבוא לידי קישוי: ולא יסתכל בבהמה ובחיה ועוף בשעה שמזדקקין זכר לנקבה. ומותר למרביעי בהמה להכניס כמכחול בשפופרת מפני שהן עסוקין במלאכתן לא יבואו לידי הרהור:‏

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אָסוּר לְאָדָם שֶׁיַּקְשֶׁה עַצְמוֹ לְדַעַת אוֹ יָבִיא עַצְמוֹ לִידֵי הִרְהוּר, אֶלָּא אִם יָבֹא לוֹ הִרְהוּר יַסִיעַ לִבּוֹ מִדִּבְרֵי הֲבַאי לְדִבְרֵי תּוֹרָה שֶׁהִיא ''אַיֶּלֶת אֲהָבִים וְיַעֲלַת חֵן'' (מִשְׁלֵי ה, יט). לְפִיכָךְ, אָסוּר לְאָדָם לִישֹׁן עַל עָרְפּוֹ וּפָנָיו לְמַעְלָה, עַד שֶׁיַּטֶּה מְעַט כְּדֵי שֶׁלֹּא יָבֹא לִידֵי קִשּׁוּי. וְלֹא יִסְתַּכֵּל בִּבְהֵמָה וְחַיָּה וָעוֹף בְּשָׁעָה שֶׁמִּזְדַּקְּקִין זָכָר לִנְקֵבָה. וּמֻתָּר לְמַרְבִּיעֵי בְּהֵמָה לְהַכְנִיס כְּמִכְחוֹל בִּשְׁפוֹפֶרֶת, מִפְּנֵי שֶׁהֵם עֲסוּקִים בִּמְלַאכְתָּם לֹא יָבוֹאוּ לִידֵי הִרְהוּר: ‏
prohibition to ensure erection.--> sleeping on his back, not observing animal reproduction.

We see that everything that may result in erection, tight pants, to lie on the back, to ride a camel (see Piske Rashbats here) fall on this category or Makshe Atsmo ladaat. To see things that are made specifically for this enter also in this category. He added in name of Tashbats that a man can judge by himself if it is dangerous for him, e.g. riding on the back of a donkey.