He should get a checkup and you should stop getting mad. If you can't stop getting mad and his checkup doesn't give an answer (or change things) then you should look seriously at whether to get married. If I had to guess, I'd hazard the notion that he is not very excited about getting married.
(But please don't get indignant if it turns out that is what's bothering him. He's only 19, almost anyone that age would be uncertain about marriage.)
He asked me without me making any kind of effort or pressure to get married. So I wouldn't understand if that's the problem
Well, you could test the idea by just asking some day if he is having second thoughts. Of course, you'd have to do that super-tactfully and carefully if he feels badly about having them [if he does]. Did he have erectile issues before you got engaged?
Yes he has throughout our relationship. We haven't had sex yet.....
Another couple of things that occur to me is that he either is closeted gay, or possibly that he has really emotional issues relating to sex. (A friend of mine who have been abused as a boy told me later that when it came time to get interested in sex, he was a wreck. "I felt like if you liked someone, then this awful thing would have to happen," he told me later.) Anyway, do you know if your fiance ever had sex with anyone?