You did the right thing by setting a strong limit and educating them about the need to be vigilant and not permit anyone to touch their genital area. If you have not already done so, it would make sense to talk with your older son about how he came to ask the boys to lick his penis. This is not a behavior that spontaneously occurs to children, though he may not recall procisely what happened.
Thank you so much! I did ask my older son and he said he does not know.....