Do Sexual Thoughts Or Sensations Nullify the Ritual Washing (Wudu)? 

Question: What nullifies Wudu?
Answer:

In the Name of Allah, Most Merciful and Compassionate,

Assalamu ‘alaykum dear sister in faith,
The release of pre-sexual emission (madhi) necessitates ritual washing (Wudu).
Sexual thoughts or sensations, on their own, without observing or feeling the release of pre-sexual emission (madhi) is not cause for the nullification of one’s state of purity.
You do not have to keep checking. However, if you do feel the wetness of such emission, then cleaning the private area and renewing the ritual washing (wudu) would be necessary before prayer.
[Shurunbulali, Nur al-Idah, pg. 82]
Allah, Most High, knows best.
[Ustadha] Iman Badawi
Checked and Approved by Shaykh Faraz Rabbani