I have joined a few couples for a threesome and it has always been made clear before hand, or easy to assume, how much guy/guy contact there will be.

I remember one threesome fail where I was told the guy in the couple was OK with contact but didn’t want to do anything ‘gay’.  As things started to get hot the girlfriend and I were 69’ing and he was fucking her doggy style from behind.  Usually I’d give his cock and balls some attention but I didn’t want to do anything ‘gay’ so I just focused on her pussy.

But, it is a tight space down there and his balls might have bounced on my head accidentally.

It freaked the guy out, he got up and left.  Left his girlfriend there 69’ing me.  We both felt bad for him so we stopped things and she left.  Man did I have blue balls after that.