I mean this isn't actually what the message of the episode is. The curse Angel was under was that if he ever experienced true happiness he would become a demon again. The sex he and Buffy had was GOOD, and beautiful, and that was actually why it triggered the curse. Angel has sex later with someone he doesn't love and it didn't cause the same effect.

Likewise when Buffy tells Giles her father figure that she had sex, and she's feeling like it was her fault that Angel became Angelus, as if having sex was bad, he says "If you're looking for judgement, you won't find it here, you will only ever have my support and my respect". And it's a really wholesome moment.

Buffy is later shown as having sex with multiple other people in the show - Parker (a casual hookup who treats her badly), Riley (which goes well although he's not the most interesting character) and Spike (who she is shown as having very passionate and aggressive sex with on her own initiation multiple times).

Another interesting thing about Buffy is that it also depicted lesbian sexuality and showed Willow giving oral sex to her girlfriend (although it was cut so that you didn't see anything explicit it's very obvious what she's doing).

Sex in Buffy isn't inherently bad, or inherently good, it's depicted in quite a diverse way, showing the varieties of sexual experiences from transcendent true love sex, to passionate animalistic sex, to casual sex, to manipulative sex, to attempted rape.