The seventh commandment forbids adultery - this includes any thought, word, or action which encourages sexual activity outside of the marriage between one man and one woman.

Exodus 20:14

14 “You shall not commit adultery.

(ESV)

Proverbs 7:4–9

4   Say to wisdom, “You are my sister,”
    and call insight your intimate friend,
5   to keep you from the forbidden¹ woman,
    from the adulteress² with her smooth words.
6   For at the window of my house
    I have looked out through my lattice,
7   and I have seen among the simple,
    I have perceived among the youths,
    a young man lacking sense,
8   passing along the street near her corner,
    taking the road to her house
9   in the twilight, in the evening,
    at the time of night and darkness.

Footnotes

[1] 7:5 Hebrew strange
[2] 7:5 Hebrew the foreign woman

(ESV)