Setting aside the logical impossibility of people f-king up but not f-king down, that is not true at all: men will f-ck down quite readily if it is convenient and there is low commitment. This is due to the obvious reason that males can have ten kids in a day by ten different women, whereas females can have perhaps 10 kids in a lifetime and each birth is incredibly high risk to her.

The core problem in the mating world is that the middle of the female attractiveness curve is having sterile sex with the top of the male attractiveness curve and then failing to pair bond with the middle of the male attractiveness curve. This is due to female hypergamy coupled with male promiscuity, arising from the dynamics of male and female reproductive constraints.

Isn't the person you f-ck up, effectively f-cking down? Or, more eloquently: without f-ck down, there is only f-ck sideways :)

I suppose it's possible for both sides to feel like or think that they're 'fucking up', or 'down', for that matter.

When it comes to social status, women tend to date up or sideways. When it comes to physical attractiveness, men tend to date up or sideways.

I don't disagree with that, but my point was more that 'dating up/down' is murky/subjective, so both sides could feel like they're dating up or down regardless of what other people might think. So even if two people were to be dating based on physical attractiveness, they could both think they got the better deal.