No. It is not a sin. Genesis 12 tells us Sarai was beautiful, even in her old age. Some woman are very beautiful and to recognize this fact is, in itself, not a sin. Recognizing a woman’s beauty does become a sin when a man lingers and lusts after that woman. It becomes a sin when he begins to undress her, stare at her and connect with her in his mind.

The first look is often not sin. It is the second look and/or the lingering look where sin enters in.