Shaykh Muḥammad ibn Ṣāliḥ al-ʿUthaymīn (d. 1421 AH) said:

The semen of the sons of Ādam is considered pure. ‘Semen’ refers to the thick fluid that is excreted from a male in response to sexual arousal. Allāh—the Most High—has described it in His saying:

أَلَمْ نَخْلُقكُّم مِّن مَّاءٍ مَّهِينٍ

“Did We not create you from worthless water (semen, etc.)?”
(al-Mursalāt, 77: 20)

This water or fluid (i.e. semen) is thick to where it does not flow easily as opposed to water that does flow and move which would not be considered worthless [due to its use in washing and drinking]. This [thick] fluid was used to create the children of Ādam (عليه السلام). The Most High said:

وَلَقَدْ خَلَقْنَا الْإِنسَانَ مِن سُلَالَةٍ مِّن طِينٍ ‎﴿١٢﴾‏ ثُمَّ جَعَلْنَاهُ نُطْفَةً فِي قَرَارٍ مَّكِينٍ

“And indeed We created man (Ādam) out of an extract of clay (water and earth). Thereafter, We made him (the offspring of Ādam) as a nuṭfah (mixed drops of the male and female sexual discharge) (and lodged it) in a safe lodging (womb of the woman).”
(al-Muʾminūn, 23:12-13)

It is from this fluid that the Prophets, Awliyāʾ [close friends and allies of Allāh], the Ṣiddīqūn [those followers of the Prophets who were first and foremost to believe in them], the martyrs and the righteous were created. We possess three methods of ascertaining its purity:

1. The fundamental state of all substances is purity. Whoever claims it to be impure must bring evidence to that effect.
2. Āʾishah (رضي الله عنها) used to scratch the dried semen of the Prophet (صلى الله عليه وسلم)¹ and would only wash it if it was still wet.² If semen was considered najs, she would not have sufficed in only scratching it when dried. As the Prophet (صلى الله عليه وسلم) said concerning the blood of menstruation: “Scratch it, then sprinkle it with water, then quench it with water, then pray in it.”³ Here, after scratching it he ordered for it to be washed. Likewise, if semen was considered najs, we would be obligated to wash it and simply scratching it off would be insufficient, just as it is for the blood of menses.
3. This water is the origin from which the sincere servants of Allāh including the Prophets, the Ṣiddīqūn, the martyrs, and the righteous were created. It opposes the wisdom of Allāh that such pious people were created from a substance that is impure.

Once a man came upon two scholars arguing with one another. He said to them: “What is the matter with you two?” One of them replied: “I am trying to affirm his origin as being pure while he tries to label it impure.” This was because one of them held the opinion that semen was pure, while the other considered it impure.

Ibn al-Qayyim (رحمه الله), in his book ‘Badāʾiʿ al-Fawāʾid’, narrates a debate that occurred between two men, one of whom held the opinion that semen was pure and the other considered it najs. The debate is of great benefit to the seeker of knowledge.

[Q]: Why is it not said that semen is impure just like urine,defecation and the rest of the excretions of the children of Ādam?

[A]:

1. Not all the excretions of the children of Ādam are considered najs. For example, sputum, mucus, and sweat are all pure.
2. There is a clear difference between urine or defecation and semen. Urine and defecation are the waste products of consumed food and drink. Their smell is putrid, detestable, and contemptible to both sight and smell. Thus, it is considered najs. As for semen, it is the opposite of these two substances as it represents the filtered, pure form of consumed food and drink. This is because [the beneficial components of] food and drink first enters the blood. This blood is then used by Allāh—the Most High—to nourish the body in its entirety. Then, in response to sexual arousal, it takes the form of this fluid [semen] from which a complete human being is created. Thus, the difference between these excretions is plainly evident in terms of the practicalities of both substances. Therefore, it would be incorrect for both types of substances to adopt the same ruling. This [semen] is a pure, filtered, unadulterated excretion while [urine and faeces] are contemptible, putrid, and widely detested.

Endnotes:
[1] Authentic: narrated by Muslim: 288.
[2] Authentic: narrated by al-Bukhārī: 229, 230.
[3] Authentic: narrated by al-Bukhārī: 227 and Muslim: 291.

Source: Al-Sharḥ al-Mumtiʿ 1: 453-455
Translated by: Riyāḍ al-Kanadī