IMO all feelings are natural. We have them, they are our feelings, we are not responsible for those, but we are responsible for our actions. Jealousy is a legitimate feeling, and should be respected as such. But it causes so many problems, and is so painful, I find it very useful to have a good thread about it. Often I think it is like fear of bereavement, but as opposed to actually loosing someone- which, however painful, you do get over someday - it does not go away! It just goes on and on, if you cannot find a way to deal with it. Yes, definitly something to be taken seriously and to work with.
Impossible to control? Do you mean as in being sad, or yelling, or as in killing your partner so no one else can have her/him? I think all violent feelings are hard to control, but with this as with others - hate, fear etc - we are asked to control them. Jealousy is just one other, if you see what I mean. I do not think it should 'rate' as being more understandable if you commit violence out of jealousy, than if you do it out of hate. In fact, violence out of jealousy is, to me, very much like hate. It certainly isn't love - not to me! Though this might be a very good point to discuss!
Reasonable behaviour? Again, I am not quite sure what you are thinking of, except that it is a feeling we get often, sometimes for realistic reasons, sometimes for our own inner reasons. I think we should be 'allowed' to show our feelings, and not have some of them put in a box called 'unacceptable'. They are there, they are our feelings and part of us, we should not be reduced to censorship or self-censorship! But on the other hand I do not think it should be cultivated or encouraged, the way the mono-culture often do. I think these problems are much better handled if treated like problems, not moral questions.