The Grand Scales

Field Spell Card
When a player's monster(s) is destroyed (except with this effect), that player must destroy 1 Spell or Trap card on the field. When a player's Spell or Trap card(s) is destroyed (except with this effect), that player must destroy 1 Monster on the field.

Notice if there are no other Spell/Trap cards on the field when a monster is destroyed, the only other target to be destroyed is the field spell card itself.

So if a monster of mine is blown up, I also lose an S/T?

"When a monster(s) is destroyed except by this card's effect: Its owner must destroy 1 Spell/Trap card on the field. When a Spell/Trap Card(s) is destroy except by this card's effect: The owner of that Spell/Trap Card must destroy a monster on the field."

this one prolly worded better. as well both effect are mandatory.

yes scheath's version is more correct. If you lose a monster, you get to blow up ANY S/T on the field, so it's usually a compensation effect.

so any comments?

it's a-go