I have seen the following and similar explanations of the Greek word translated to lust on several trustworthy Christian sites:
The most common words translated as lust in the Bible are epithumeo, epipotheo, and orego. They all mean to desire, to covet, to long for intensely, to set one’s heart upon. This word is actually used in a positive sense in Matt. 13:17; Luke 22:15; 1Tim 3:1; Heb. 6:11; and 1Pet. 1:12. It is also used in a negative sense in Matt. 5:28; Rom. 7:7; 13:9; 1 Cor. 10:6; and James 4:2.
The NIV's is an unfortunate translation because the word epithumeo appearing in the verse is a verb (to lust) not an adverb (lustfully). The operative verb is not look, but lust. Jesus is not talking about how a man looks at the woman or thinks about a woman, but about a man lusting.