>Is there any difference between the two that's relevant legally
Yes, the difference between how gay marriage would affect the risk of HIV transmission vs. how sibling marriage would affect the incidence of genetic defects in babies.
>Your argument above made some sense when distinguishing overall gay sex increasing from risk increasing, but you seem to have abandoned that in your last sentence
I'm not sure what you mean. Gay marriage will neither increase the total amount of gay sex nor increase the risk of HIV transmission. There is simply no connection between HIV and gay marriage, so it would make no sense to try to use HIV to justify a ban on gay marriage.
>I could say the same about sibling marriage for you
You could, except that it wouldn't be true. Having children outside of marriage is still a big deal for a significant number of people. Unprotected casual sex is, virtually by definition, not something that appeals primarily to people who want to get married. Again, the facts are important. You can't just make up crazy hypothetical scenarios and use them as the basis of your argument.
Yes, the difference between how gay marriage would affect the risk of HIV transmission vs. how sibling marriage would affect the incidence of genetic defects in babies.
>Your argument above made some sense when distinguishing overall gay sex increasing from risk increasing, but you seem to have abandoned that in your last sentence
I'm not sure what you mean. Gay marriage will neither increase the total amount of gay sex nor increase the risk of HIV transmission. There is simply no connection between HIV and gay marriage, so it would make no sense to try to use HIV to justify a ban on gay marriage.
>I could say the same about sibling marriage for you
You could, except that it wouldn't be true. Having children outside of marriage is still a big deal for a significant number of people. Unprotected casual sex is, virtually by definition, not something that appeals primarily to people who want to get married. Again, the facts are important. You can't just make up crazy hypothetical scenarios and use them as the basis of your argument.