1. "I don't think its ever going to happen."
When women started campaigning for the right to vote, there may have been men who were a little sympathetic to their cause; but being aware of the degree of opposition by most men, would doubt that it would ever happen that the law would change and allow women to vote. "I don't think its ever going to happen." Its an opinion, based on weighing up the facts as the person sees it, but the person could equally have said, " I may be wrong, but I don't think its ever going to happen. I just can't see most men agreeing to allow the law to be changed."
2. "It will never happen."
This is an outright assertion. The person is quite definite. He has no doubt in his mind. If someone in the late 19th century/early 20th century had suggested that women might one day be soldiers in the army, he would have been laughed at - "It will never happen." (Now, British women have not only long been in the armed forces, they are fighting in the Army over in Iraq.) So, the person is definite in his opinion...but it doesn't mean the person is right!