I'm coming to this way late, but I remember the rationale for using a condom with JC and any other vampire/lycanthrope with whom Anita fucks. Though both vamps and weres can't catch STDs, if they recently had sex with someone who had an STD the virus or bacteria can be transferred to another partner. That is, if the creepy crawly that causes the STD is still alive on their penis, it could infect another partner.

At the risk of defending LKH, Anita was on the pill even before the pregnancy non-scare in DM, but she only used condoms with new or unfamiliar partners like Byron (in ID?).