He proceeds from utilitarian premises, which require you to evaluate ethical decisions in terms of pleasure and pain. I'm going to lay this out into separate numbered clauses so you can see it.
1a ...the presence of pain is badNotice that he only adds an "only if" condition to 2b, not to 1b. He calls this an asymmetry.
1b and the absence of pain is good,
2a but whereas the presence of pleasure is good,
2b the absence of pleasure is bad only if somebody is deprived of that pleasure.
Really, it seems like all evaluations of pleasure and pain and their absence are true only if someone is there to experience it. The "only if" actually applies to all 4 of these statements.
But, I'm not actually a utilitarian, and I think life is the fundamental good, so I can't go along on this ride anyway. But if you do go along on the ride, he then argues:
3a If nobody is deprived of an absent pleasure –In other words, a world of pain and pleasure is a mixed sum of good and bad, but a world with neither is an absolute good.
3b because the person who would have experienced the pleasure never existed –
3c then the absence of that pleasure is not bad.
How cruel is evolution, which created this unpleasant
sensation to warn us when hazard is present.