Fidelity to G-d

דַּבֵּר אֶל בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם וְעָשׂוּ לָהֶם צִיצִת עַל כַּנְפֵי בִגְדֵיהֶם לְדֹרֹתָם וְנָתְנוּ עַל צִיצִת הַכָּנָף פְּתִיל תְּכֵלֶת

וְהָיָה לָכֶם לְצִיצִת וּרְאִיתֶם אֹתוֹ וּזְכַרְתֶּם אֶת כָּל מִצְוֹת יְקֹוָק וַעֲשִׂיתֶם אֹתָם וְלֹא תָתֻרוּ אַחֲרֵי לְבַבְכֶם וְאַחֲרֵי עֵינֵיכֶם אֲשֶׁר אַתֶּם זֹנִים אַחֲרֵיהֶם

The Torah says "Do not stray/turn after your hearts and eyes."

Ramban learns that "hearts" are a reference to apikorsus while "eyes" are referencing znus.

The verb used to describe lusting after these things is going back on both the sin of the eyes as well as the sin of the heart.
For, znus, is something which can pertain to our relationship with Hashem.

Ramban in Exodus 20:2 says that
G-d is "kel kanah."

The reason for Hashem's jealousy is because we the Jewish people are Hashem's chosen nation. If G-d's nation, G-d's spouse, were to turn to other gods Hashem will be "mikaneh" His spouse just as a husband warns his wife suspected of infidelity.

You see that the relationship we have with G-d is that of being a spouse.
Remaining committed to our spouse by keeping his Mitzvah and not being guilty of infidelity by worshipping other gods.

The result is that cheating on G-d, is zenus.
Infidelity towards G-d is adultery.

Therefore the verb "zonim" is highly appropriate.

Ibn Ezra sums it up...
אשר אתם זנים - כי מי שילך אחרי תאותו הוא זונה מתחת עבודת אלהיו