Reference: Fataawa an-Nadthar wal-Khulwah wal-Ikhtilaat – Page 10
Question: Why has Islaam forbidden shaking hands with women who one is not mahram to?
And does one’s wudhoo became nullified if he shakes the hand of a woman without desire?
Response: Islaam has forbidden that [shaking hands with women who one is not mahram to] because of the fitnah [which exists therein] and the greatest fitnah is for a man to touch the skin of a woman (ajnabiyyah); and everything which leads to any fitnah has been legislatively forbidden. Therefore, one is commanded to divert his sight as a preventative measure. As for one who touches his wife, then this does not nullify the wudhoo. Even if it is done with desire, until any prostractic fluid (mathee) or semen (manee) is ejaculated. So this then obligates him to perform ghusl if only semen is ejaculated and if it is [simply] mathee (prostractic fluid) then it is sufficient to wash the private parts and make wudhoo.