Exactly this, imo any rubbing between 'the wet bits' is an absolute no and no different to penetrative sex.
I don't often disagree with you, but it will perhaps come as no surprise, that I do on this.
The OP question was 'comparative' risk. If we exclude from the discussion those STIs that are transmitted from skin to skin, then in terms of those spread by bodily fluids, then both activities carry a risk due to potential exchange.
However the risk of successful transmission from the fluids of an infected individual is, in most instances, considerably less with genital rubbing than with unprotected penetration. This is to do with quantity, duration, the effects of the physical nature of the sex act and the environment in which it takes place. Both urethras are more at risk from penetrative sex than external genital rubbing.
Also people tend to assume that if you do have sex with someone who is positive, then you will automatically become infected too.
If we take Chlamydia for example, then the chances of transmission
from a single exposure may be as low as just 5%. This figure could rise to 30-40% in some cases, but that is still far from guaranteed transmission.
If you combine that statistical risk with the other above factors, then while genital rubbing is certainly not safe, it is generally considered to be a low risk activity.
Then of course as always with this discussion, there is the 'elephant in the room'
