Question Summary:
Waswasa During Ghusl

Question Detail:

I was doing ghusl and I was taking too long, causing me to get frustrated and say a thing that a kuffr will say to Allah SWT .
This is a stupid question but is my ghusl valid in this case?
If someone has thoughts about the opposite gender during ghusl, is it valid?

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykumwa-rahmatullāhiwa-barakātuh.
A believer should always maintain the greatest amount of respect for Allah in his heart. He should never show any disrespect to Allah even in the various challenges of life. Contain oneself and regard the challenges of life as a test of ones Iman. 
If you habitually take long for gusl, that is abnormal and a sickness. You should consult an appropriate specialist to diagnose your disorder and prescribe for you medication. That will remove your frustration and save you from inappropriate utterances. In any case, the gusl will be valid.
In gusl, concentrate on cleansing the body and be more concerned of purifying oneself internally. That will help avoiding thoughts of the opposite gender. 
And Allah Ta’āla Knows Best
Zakir Husain
Student DarulIftaa
Michigan, U.S.A
Checked and Approved by,
Mufti Ebrahim Desai.