אבן העזר כ״גEven HaEzer 23

Se’if 1א׳

אָסוּר לְהוֹצִיא שִׁכְבַת זֶרַע לְבַטָּלָה. וְעָוֹן זֶה חָמוּר מִכָּל עֲבֵרוֹת שֶׁבַּתּוֹרָה. לְפִיכָךְ לֹא יִהְיֶה אָדָם דָּשׁ מִבִּפְנִים וְזוֹרֶה מִבַּחוּץ, וְלֹא יִשָּׂא קְטַנָּה שֶׁאֵינָהּ רְאוּיָה לֵילֵד:

It is prohibited to spill seed needlessly and this sin is more severe than all Torah transgressions. For this reason a man should not thresh inside and sprinkle [his semen] outside [of a woman], and he should not marry a girl who is unable to have children.

Se’if 2ב׳

אֵלּוּ שֶׁמְּנָאֲפִים בַּיָּד וּמוֹצִיאִים שִׁכְבַת זֶרַע, לֹא דַּי לָהֶם שֶׁאִסוּר גָּדוֹל הוּא, אֶלָּא שֶׁהָעוֹשֶׂה זֶה בְּנִדּוּי הוּא יוֹשֵׁב, וַעֲלֵיהֶם נֶאֱמַר: "יְדֵיכֶם דָּמִים מָלֵאוּ" (יְשַׁעְיָה א, טו), וּכְאִלּוּ הָרַג הַנֶּפֶשׁ:

Those who 'commit adultery' with their hand and thereby cause semen to be spilled it is not enough that this is a great prohibition, but one who does so should sit in excommunication and about them it is said "Their hands are full of blood" and it is as if they have killed a person.

Se’if 3ג׳

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:

It is forbidden for a man intentionally make himself have an erection or to cause himself to think about sex. Rather if he thinks about sex he should remove himself from vain things and go to the words of Torah which is like a beloved doe and a graceful deer. Therefore it is forbidden for someone to sleep on their back with his face up, rather he should lean to the side so that he doesn't come to have an erection. And he shouldn't look at a beast or a wild animal or bird when the male and females are copulating. But it is permitted for those who bring animals to mate to 'put the applicator in the tube' because that is their work and it won't cause them to have sexual thoughts.