Even the most confident person has to have moments of self-doubt. He may also fear for your safety - a valid concern. It could also be a lack of understanding on his part. It could be something he's never thought about - he may have just accepted the popular opinion of his upbringing. It's probably a combination of all of the above.
Sexual orientation is a controversial issue.
It is difficult to separate opinion from fact, and acceptance does not come from many sources. It sounds to me like he's got more of a trust problem than a sexuality problem. All I can suggest is that you give him no reason to distrust you. And if he still acts that way, I'd consider that a warning sign of possible possessiveness, which can be a real problem. It's probably also affordable.