Sforno on Deuteronomy 4:3/ספורנו על דברים ד׳:ג׳

1עיניכם הרואות, Moses reminds the people that they had observed the truth of this with their own eyes when their brethren, in their lust, not meaning to commit idolatry, had nevertheless done so, as they had been overcome by their libidos at the time so that they succumbed to seduction. The Torah testifies that their original sin had “only” been to whore with the daughters of Moav. (Numbers 25,1) The Torah, in outlawing sexual relations with non Jews had added that if this law was ignored the result would be intermarriage, idol worship by the children of such a marriage, etc. (Exodus 34,16) Every Israelite who indulged his lust for a Moabite girl had considered himself immune to such a warning. They proved to have been wrong, with the loss of 24.000 Israelites as a result. G’d had wiped out everyone of them who had worshipped Baal Peor. They had been tricked into it without realising it.