Response: Islaam has forbidden that because it is a temptation. One of the greatest forms of temptation is for a man to touch a woman he is not related to. Everything that leads to temptation is prohibited by the Law. This is why one is required to lower one’s gaze as a means of blocking that evil.
As for a man touching his wife, it does not invalidate the ablution. This is so even if it is done with lust – unless he releases some prostatic fluid or sperm. In that case, he must make ghusl if it were sperm; he must make ablution and wash his male organ and testicles if it were prostatic fluid.
Shaykh Ibn ‘Uthaymeen
Fataawa al-Mar.ah fatwaqonline.com