This is similar to a driver driving a car on a straight immaginary road start to end without any difficulty with speed and thrill but if he drives on zigzag road he has to drive slowly, carefully, with stoppage and discomfort.
Straight love means face-to-face, love-to-love, mother-to-child.
A mother completely devoted to the child with with love and care.
This painting is done mother and child is always undeviated which is like a straight line.
This painting is done by straight lines.