Is your father marrying a woman? Why would she think he's gay? I think you need to be assertive with her. This is your father and you can say - you don't want to hear negative things about your dad and you can make your own decision about going to the wedding. She has no right to tell you not to go or try to build a nasty wall between you and your dad. Even if he was gay, this is his life and this has nothing to do with him being a father. Don't let her manipulate you and you don't have to turn against your father to appease your mother - you shouldn't have to choose and after 25 yrs, she needs to mind her own business.