Please excuse me, but this has nothing to do with homosexuality and that is a very ignorant comment ... I think that it is (overall) normal childhood curiousity and depending on the maturity level etc. of the 8 yr. old, he may not FULLY understand how inappropriate these episodes are.
I can't help but also feel that the 6yr. old may have instigated these episodes though. Especially where he has some issues, possibly even in this area? (and your son doesn't, and you are open with him) I know you don't want to forbid interaction for fear of sending the wrong message, but honestly, if it were my 6 yr. old son, I'd minimize their play time ... even supervised. And I'd probably get him some counseling. Relax mom, don't beat yourself up! You seem to be loving, caring parents and in most cases, that is all a child needs to be (mentally) healthy.
Good luck!
you need to be very concerend with this situation and take things into your hands, only if you do not want a gay son.
You already know that close supervision is required, particularly during play between the two boys. While you know of the other occasion, my guess is that this type of behavior has occurred on other occasions with the two boys as well. When such a thing occurs, it is always sensible to inform the other parent. This is information that parents need to have so that they can respond to it. You are setting sound limits - i.e., no touching others and no allowing others to touch you. While therapy per se may well not be necessary, it might help to schedule an appointment with a pediatric mental health clinician who can evaluate your son and offer you guidance. You might well rest easier if you follow such a path.