What exactly is the sin of lust?
What distinguishes lust from the unadulterated sexual impulse, is that lust involves the feeling that one wishes to
ravish the other, or be ravished
by the other. (Both are equally sinful.) There is an element of rapaciousness present — much though it is taboo to admit this. Lust is (therefore) also the enjoyment of a
loss of self-mastery, the delight of casting aside restraint and indulging the desire to “have” the other, or to “be had”. The lustful sexual impulse can arise only when the other is perceived — even if only temporarily — primarily as a
body, a tangible shape that can subdue (or be subdued by) another body. The unadulterated sexual impulse, on the other hand, is the desire to
unite, which requires perception of the other as one’s perfect, living, divine complement.
As for “looking”, it depends very much on what you see when you look. Do you see flesh? That’s lust. Do you see one who can complete you? Then it might be love. But any intense, longing looks (at someone who isn’t your spouse) are suspect, because they are immodest. So be wary of deceiving yourself (or the other) into labeling something “love” to eagerly. Sexual attraction is an extremely strong impulse, and we tend to justify it too easily.