Testicles chafing on legs - are my bibs too loose or too tight?
"we're all adults here..."

In my experience, when riding with bib shorts or cycling shorts, they have to be pulled up, way up.  And they should be a snug fit around your lower body - the shoulder straps are more forgiving.

Ideally your pants cloth should be in contact with your skin all the way up your thighs and right up to where the skin turns around and comes back down as the scrotum.  (sorry I can't find a medical name for this crease in the skin.)

This should leave two layers of cloth between the boys and the thighs.  

Chamois cream might help, but you won't know till you try it.  Vaseline / Petroleum Jelly is worth trying if you already have some (most households do) but its not as good as the proper product.