Short answer
Thoughts and looking do not break wudu by themselves, and the four schools agree. But arousal often releases madhy (pre-seminal fluid), and madhy does break wudu. If semen comes out with desire, ghusl is required. Looking at haram is a sin of its own even when wudu stays.
Desire, thoughts and looking do not break wudu. Madhy that comes out because of arousal breaks wudu and must be washed off. Semen that leaves with desire requires ghusl.
The four schools agree on this.
Explanation
Wudu (ablution) is broken by what leaves the body and by a few other set causes, not by what passes through the mind. So a passing thought, a dream you remember while awake, or an image you saw does not by itself break wudu. The four schools agree on this.
What matters is what happens in the body. Arousal often releases madhy: a thin, clear, sticky fluid that comes out without spurting, often without you noticing. Ali had a lot of it and sent someone to ask the Prophet ﷺ, who told him to make wudu and wash his private part. So:
- Thinking or looking, with no fluid: wudu stays.
- Arousal with madhy: wudu breaks. Wash the private part and the spot on your clothes, then make wudu again.
- Semen leaving with desire: ghusl is fard. See when ghusl is required.
- Not sure whether anything came out: wudu stays until you are sure. Do not keep checking.
For the difference between madhy, semen (mani) and wadi, see precum and ghusl. Touching your spouse does not break wudu in the Hanafi school either, unless fluid comes out or there is direct contact of the private parts: see touching a woman.
Wudu staying valid does not make the act fine. Watching something haram is a sin that needs repentance, and Allah tells believing men and women to lower their gaze. Wudu itself is a help here: each time you wash, sins of the eyes and hands fall away with the water, as the Prophet ﷺ described.
The evidence
Quran, Surah An-Noor 24:30
قُل لِّلْمُؤْمِنِينَ يَغُضُّوا۟ مِنْ أَبْصَٰرِهِمْ وَيَحْفَظُوا۟ فُرُوجَهُمْ ۚ ذَٰلِكَ أَزْكَىٰ لَهُمْ ۗ إِنَّ ٱللَّهَ خَبِيرٌۢ بِمَا يَصْنَعُونَ ﴿٣٠﴾
Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do.
Lowering the gaze is a duty of its own, apart from the rules of wudu.
Sahih Muslim 303
Sahih... توضا وانضح فرجك ...
"Make wudu and wash your private part."
The Prophet's ﷺ answer about madhy: it breaks wudu but does not require ghusl.
Where the ruling is recorded
- al-Hidaya, Kitab al-Taharat, chapter on ghusl. Madhy and wadi require wudu, not ghusl.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Taharah, nawaqid al-wudu. Madhy from desire breaks wudu.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.