Why does Bernard regret having sex with Lenina ?
Bernard is disgusted with society's casual sex philosophy. He wants something more than just casual sex with any girl he dates. He thinks that there might be something to be experienced by holding back one's desires and waiting for a better time to express one's sexuality with a partner. He and Helmholtz have not been dating with the intent to discover what might be the result of self-denial. Bernard tries to explain his feelings with Lenina on their date, but she is simply too conditioned to understand anything outside of what popular culture accepts--that is hedonism. For example, Bernard says something that Lenina doesn't understand: ". . . to try the effect of arresting my impulses" (93). She tells him that it seems like nonsense to her and Bernard replies with the following:
"It suddenly struck me the other day . . . that it might be possible to be an adult all the time" (94).
When Lenina says again that she doesn't understand what Bernard is talking about he says:
"I know you don't. And that's why we went to bed together yesterday--like infants--instead of being adults and waiting" (94).
Bernard is disappointed with himself that he didn't act like an adult; he didn't wait with Lenina; and he didn't uphold the ideals he and Helmholtz were experimenting with. Essentially, Bernard regrets going to bed with Lenina because it was like taking a philosophical step backwards from what he was striving to accomplish within himself.