It is clearly haraam to approach zina. There is a good response on the definition of zina.
However, it seems very difficult and impractical to completely block off every sense from the opposite sex. Eventually you will have to talk to members of the opposite sex, look at some of them in the eye, listen to them, or talk. Especially if interviewed or at the workplace!
I've been taught that it is fine to touch or look at someone if it involves healing or teaching. But what if listening to a teacher leads to fantasies? Would it be haraam to continue learning with that teacher?
Would actions between people who will never possibly physically approach each other be constituted as approaching zina? For example, long distance calls or intimate text conversations.
Especially in less Islamic nations, it is not practical to simply avoid all contact with the opposite sex or to tell them to cover up. How does one reach the balance between finding a practical way to avoid zina and not completely avoid people?