1. Organ music is always sad and serious. As with any other instrument, so the organ repertoire has the works of the most varied moods and characters. Incidentally, even the sad pieces have the delicate balance between minor and major tonalities which create those moods. Organists usually try to maintain this balance in recitals. Listen to The Gigue Fugue of J.S. Bach. There is no doubt that this fugue is really extremely fun piece both to play and to listen to. Pay attention to reaction of the audience.
2. Organ music can only be sacred. Perhaps today it seems strange, but the organ's origins were of secular nature in Ancient Greece. Only in the medieval times the Church adopted this pagan instrument in its liturgy. However, even later organ music, a large part of organ music is secular, like this Finale from the Symphony No. 1 by Louis Vierne.
3. Sacred organ music is always sad. Although the music, like the liturgy of the church often has a serious mood, it would be wrong to assume that it is necessarily sad. Even the works written in minor keys often express other feelings and characters. Very often it depends on the text. For example, it can be pieces of solemn and even dramatic character. In fact, choral prelude In Dir ist Freude by Bach is probably one of the most joyful of his choral preludes.
4. Organ music is always difficult to understand. Such very advanced music exists, there is no doubt about it. But this is only one side of the vast organ repertoire spanning 7 centuries. A number of works written in the 20th and 21st century are like that. However, even this kind of music can be properly understood and appreciated if the listener found out a bit more about the composer, his style, the structure of the piece and so on. In this case, the saying "appetite arises with eating" fits perfectly well. I think that you can befriend advanced music gradually.
5. Organ music is not suitable for young people to listen to. It probably depends on the particular piece of organ music. For example, the piece "Penguins Playtime" by the English composer Nigel Ogden sounds like an excerpt from the movie. Here you will surely find a lot of influence of pop music, which is liked by a lot of young people. On the other hand, not all of young people like pop music either, some of them are heavily into classical and art music.
6. Organ music is played only at funerals. This impression may occur to some people due to the fact that these people associate the church only with funerals (apparently they come to the church only on this occasion). While at the funeral organ music is indeed very prominent, none of the above examples are designed for this occasion. It's not clear to me why organ is not necessarily associated with joyful and sweet weddings because that's surely another side of the instrument that general public is aware of (perhaps because nowadays a lot of other instruments can play at weddings - not so much during funerals).
Proper understanding of these myths will clarify some people's reaction to the organ. Hopefully you will be able to explain in some way what organ music is all about. Perhaps then some people will no longer find it scary, sad, boring, or depressing. Most of all, organ music is the food for your brain (besides your soul). And this is a good thing, isn't it?