How to make a swing swing straight?
A triangular wooden structure as you describe should work if the ropes are the same length and the hooks the ropes are attached to are both hung vertically. Pendulum period depends on rope length, so the set up you currently have couples pendulums having two different periods. That's a recipe for unstable swinging.
Also, the angle pictured on the left attachment point will cause wobbling even if you correct the length problem. That's because it causes one rope to oscillate on a different axis* than the other.
For maximal directional stability in swinging, you want the ropes attachment points to the limb to be set a few inches wider than their attachment pints to the seat of the swing

*Actually given the hookup, a different set of axes, but that overcomplicates things. Just work to preserve symmetry between the two ropes and all will be well.