Lust for anyone outside of marriage a sin of adultery.
Adultery means:
>1. A married man having sex with another woman who is married to someone else. (Jer 29:23, etc).
>2. A married man having sex with another woman who is unmarried, including harlots. (Prov 6:32; Jer 5:7).
>3. A man lustfully desiring another woman, irrespective of her marital status. (Matthew 5:28).
>4. A man who marries a (wrongly) divorced woman. (Matthew 19:9).
>5. A wrongly divorced man who marries another woman. (Matthew 5:32).
The Greek term translated “woman” in the passage was γυνή (gynē), which means a woman of any age, whether a virgin, or married, or a widow, a wife, or of a betrothed woman.
It was translated as women more often than as a wife: as women 129 times; as wife 92 times.
https://www.blueletterbible.org/lang/lexicon/lexicon.cfm?Strongs=G1135&t=KJV
Lust for your own spouse in marriage is fine. Even desired.
https://dalrock.wordpress.com/2016/12/21/like-a-rutting-buck/
>Contrary to the argument that sexual passion is sinful if it isn’t carefully constrained, husbands are exhorted to be intoxicated with passion for their wives. The proverb offers the example to follow of a buck in the rut. For how else can we interpret this exhortation?