Specialists in childcare will tell you that a certain amount of sexual experimentation is NORMAL for prepubescent males and females. In a non-threatening way, ask your 6-year-old who proposed what and if there were any type of threats made. Then just say that this behavior is not very hygienic and you feel that it would be safer for him not to repeat this behavior.
Mom is right, it's very unusual for a 6 year old to know this might be something pleasurable to do, so the first thing as she said is to try to figure out how they figured out to do this. And then when you find that out and if you can eliminate the really bad possibilities out there, do explain that this is inappropriate but make sure you don't do that in a "from now on" way because at some point in their lives they will be old enough to do such things and you don't want to scare normal sexual development out of them, just the abnormal and too young stuff. If this is too hard, and it seems hard, you might consult a pediatric psychologist on how to go about explaining this in a way that doesn't stick to them when it is an age appropriate thing to do. Peace.
How do you know this occurred? My first concern is that something has happened to one of them by someone else. Child on child molestation occurs and is a cycle where then the child that had it happen to them by someone else then recreates the situation elsewhere. Or did one of them see something that put this in their minds. Porn or in person oral sex. Kids do experiment and perhaps they came to this activity on their own, but you really need to know to make sure that there has been no abuse that has taken place, that sexual content is not being seen by them, etc. Then I think you don't discipline this situation now but make a rule. We don't touch other people's private parts and they don't touch ours. Rule. Break the rule, consequences like all other rules. You don't really have to make a big deal out of it other than to make sure that they are both safe, both not exposed to inappropriate content for their age, and both know that this activity isn't allowed from now on.