Yeah, I'm baffled about that one too, given that monogamy is often the point of marriage. (Not so for polyfolk, obviously.)

Further, I'd think that the belief that one might someday commit to a lifelong monogamous relationship would be an encouragement to be more careful about HIV.

No, I think ilcylic nailed it: there's no logic to the argument, it's just a scare tactic aimed at paving the way for further anti-civil-rights legislation.