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!