Sforno on Numbers 25:1/ספורנו על במדבר כ״ה:א׳

1ויחל העם לזנות, originally, there had been no intention of committing idolatrous acts at all. All that the males had intended was to indulge their libido with the womenfolk who made themselves available. However, these people fell victim to precisely the warning of the Torah in Exodus 34,15-16 of what would happen if Jews would allow the Canaanites to remain in their country and conclude a covenant with them. They would be invited to their social gatherings resulting in their eating forbidden foods, and eventually intermarriage followed by lip service to their gods.

2ויאכל העם וישתחוו לאלוהיהן. This was a classic demonstration of how the evil urge works, first suggesting minor infractions of Torah law and then, gradually, suggesting major sins. (compare Shabbat 108)