“The checking themselves out wasn’t about ego. It was more of an appraisal. They were sort of looking at themselves as they really were.”
Admit it — you wondered about this too. When did humans realize that sex makes babies?
A golden oldie, This Be Close Reading:
To fuck something up is to conjure up, or cause to come into existence, by means of fucking. An example with another common verb will put to rest any doubt: for do we not say that one can whip up a PowerPoint presentation, cookies, or military intelligence, meaning thereby that we will create them by means of whipping, respectively, generic business cliches, eggs & flour, or hapless innocents? Thus we see that Larkin here refers to nothing more than biological processes at their most crude: they fuck you up, they beget you.