Can a wife kiss her husband's privates?

Yes, in Islam, a husband can touch his wife's private parts and she also can touch his private parts including holding his penis. Oral sex is allowed between husband and wife. However, both partners, wife and husband, should be satisfied and enjoying and feeling pleasure. Nothing in the sex relation between wife and husband could be done without acceptance and satisfaction of both partners.

It is forbidden in Islam to make sex with wife through inserting the penis in her anus. Also not allowed to perform intercourse during wife menstruation.

See the related question for more information.

I am not sure about other religions in this regards.