Abstinence rings have a new design: the Hebrew verse from Song of Songs "I am my beloved's and my beloved is mine" (אֲנִי לְדוֹדִי וְדוֹדִי לִי, הָרֹעֶה בַּשּׁוֹשַׁנִּים.) a common art and jewelry theme sold to Israeli tourists and in Jewish bookstores, especially common on wedding parephenalia. I can see how it's appropriate, but it's jarring.
Others they could consider. . .
אֲנִי לְדוֹדִי, וְעָלַי תְּשׁוּקָתוֹ. "I am my beloved's, and his desire is for me [but we're going to get married first]." (7:11)
For a longer verse, they could consider
עַל-מִשְׁכָּבִי, בַּלֵּילוֹת, בִּקַּשְׁתִּי, אֵת שֶׁאָהֲבָה נַפְשִׁי; בִּקַּשְׁתִּיו, וְלֹא מְצָאתִיו.
"On my bed in the nights I sought the one my soul loves; I sought him, but I didn't find him." (3:1)
as a threat about the results of inappropriately intimate dating.
There is no בבתי קפה בלילות בקשתי
"In cafes at night I sought..."