I agree - Tom didn't tell on Sonny because the Don was sharp enough to figure it out. That was Vito's subtle way of telling Sonny that type of behavior would not be acceptable if Sonny ever wanted to take over the Family.

In the novel, it stated that Vito was straight-laced when it came to sex and frowned upon Sonny's behavior. Also in the novel, Sonny & Lucy had been flirting the entire week leading up to the wedding. And Vito was furious when he found out that Fredo was womanizing heavily while in Las Vegas.