Dominance, Alphaness , masculinity, primal instinct for attracting mate.

Speaking for myself — I am in my 30s and I still think about penis size every day. More along the lines of: “How are men supposed to fit everything into their underwear?” “These pants reveal way too much.”

I just assumed everyone was curious and thinking about size all the time. Maybe it’s just a result of having nobody to talk seriously about this stuff to.

Up to a point it's along the same lines as "why are men so obsessed with football", because it's big, in your face and requires no thinking or interpretation. And to comment on my own brand new account, I wanted to comment on something else but chanced upon this :) Reproductively it probably meant nothing at all in the life-long marriage context, but in the on-off, one night society of ours it might well be an advantage. Also, we know quite well it's not really much of a pleasure or love predictor, but it's anecdotally at least a non-monogamy predictor, both owners and recipients are likely to include such adventures in their bucket lists!