Well, to answer that we need to know what agreements did you have in place when he began the new relationship, and what justification do you have for changing the rules? When did he start the relationship in relation to the decision to get married? What's changed since he started seeing this other person?
Keep in mind that when he started seeing the other person, there were rules and expectations in place. Changing those is potentially unfair not only to him, but to his partner, who is likely an innocent bystander in this.