In fact, it seems the important factor is the number of degrees between the two, not whether they share sign or element.
A trine can also be out of sign.
When day is thought of as the opposite of night, it is because of its temperament. Night is cool and dark. Day is warm and bright. Not because there is any geometrical degrees to be considered.
In Astrology also, the reason two signs oppose each other is because of their temperaments, not simply because of the degrees or geometry, which is hardly a basis by itself to consider two signs in opposition.
Astrology is based upon the temperament, element and energies of signs and planets. That is the fundamental underlying concept and extremely important to understand. This is exactly the reason why we say that Venus is weakened in the hot and dry Aries. Therefore the energy of Venus, which supports partnership (two) is lost in the sign of individuality of Aries.
In Taurus, when the cold season is gradually turning to warm spring and nature is beginning to come back to life, Venus, which is all about beauty, tranquility and nature thrives in Tau. However, it is lost in the sign of Sco, where the temperament changes to cold, wet and dark perfectly reflecting transformation and endings. The flora starts withering away and loses its colour and beauty. These are the reasons that these signs oppose eachother because their energies are in opposition. Not simply because they are 180* apart. This is why understanding the fundamental concepts behind those planets and signs is so important, especially for those that practise Horary, where especially we talk about concepts of dignities and debilities.