Or they internalize the abuse and take riskier and riskier behaviors as they perceive themselves increasingly devalued and estranged from loved ones, or the abuse can trigger latent psychological issues that might not have been actual problems otherwise. They may even perceive the STD as something of a “win” or wash if their partner contracts it from them.

Maybe. I could also see domestic abusers and their victims typically coming from more traditional and/or less educated backgrounds, hence, worse sex education, if any.

Seems a reasonable explanation. There may also be other factors such as a partner harbouring resentment if they think they contracted HIV from them.