I am an 18 yr old girl, so i don't know if i would be considered a teen anymore... but i like your way of answering questions so i thought i'd give it a shot.
I have a friend in my church who is asexual, because of biological and mental issues. Not allot of people understand his condition, so he doesn't have many friends. I'm familiar with it, and we get allong well, so i've become his best friend I guess. My parents insist that he has a crush on me, or is falling in love with me, or is attracted to me and all such nonsense. I've talked to them about it, and they say he's just lying to me about being asexual because he know's i'm engaged. I've showed them the facts about his condition, and they still don't belive me. My goal is to make my parents stop gettin all upset over it. Even if he does like me, he's been very respectfull, and not foward at all. I don't see anything wrong with it. Do you have any ideas on how I could make them understand where I'm comming from, how i feel, he's not attracted to me, and even if he was, it's ok?
ANDY'S ANSWER: Not a lot of people are even familiar with the term "Asexual", so just to clarify, an asexual is someone who is not sexually attracted to either males or females. They are still capable of having and enjoying sex, but are not driven to do so.
Even though your friend is not sexually attracted to you, he still has the capacity to love you deeply. I'm sure there are lots of people in your life that you love and who love you, and it doesn't involve a sexual relationship. This is one way that you can explain it to your parents, however what is more important is how your fiance feels about this relationship. Jealously occurrs on many levels, not just because of sex. Intimacy, even when it is on a friendship level, may be more than your fiance can compete with. Just to be clear, the same situation could be complicated by a best friend who is a woman if your fiance believes that his relationship to you is threatened by a closer relationship with someone else.
Also, to the outside world, your friend is still a man. Most people do not really believe that there is such a thing as a asexual person, so there can be doubt cast on your motives for cultivating a relationship with another man, (after all, presumeably, YOU aren't asexual) as well as questions and pressure on your fiance from his friends and family who are concerned about him getting hurt.
Human relationships are complex at the best of times and most people feel uncomfortable when something does not fall within their normal perimeters of expectations. My advice would be to take this up head to head with your fiance and resolve any issues that he may have. If he is fine with your friendship, no one else really has anything to say about it. If he is not fine with it, then it is you who will have to decide which relationship is more important to you.