Buffy and Riley use condoms during their love making. It's nice to see a show enforce safe sex; Anya also mentions using condoms in "The Harsh Light Of Day". We didn't see Buffy use condoms when she slept with Angel, but it is a common belief in vampire mythology that they're sterile and cannot bear children. STDs might be a concern with vampires, but some sources take the view that vampires cannot be carriers for human diseases. Finally, it could simply be that we just didn't see the condom usage between Buffy and Angel.