Toddler Boys touching each other inappropriately

[sas0828]

My 10 year old son recently found my recently turned 5 year old nephew and my almost 3 year old son in a private area.  When he asked what they were doing, my nephew was obviously fibbing, but when my son promised not to tell his mom, he admitted to my son that he and my 3 year old son were touching each other & licking each others butts and "weiners".  Of course I told the babysitter to make sure they weren't going off alone in the bedroom to play and to keep a closer eye on them, but is this normal curiosity or should I be concerned.  We told our son that he's not allowed to do this anymore and that if his cousin wanted him to that he needed to tell him no and get an adult, but he's only 3 and I'm not sure that he will be able to do this.  I have absolutely no concern that either of the boys are experiencing any abuse, but I'm still concerned and want to make sure this behavior stops.  I do remember my now 10 year old son going through this sort of stage, but it was with the opposite sex.  Is this a sign of being homosexual or typical curiosity for both a 3 and 5 year old?