They might; something being officially sanctioned might increase the frequency.
Can you at least see a reasonable comparison between the two? If you don't think changing marriage laws affects behavior, shouldn't that apply to incest as well?
It seems to me that "legalizing gay marriage won't increase gay sex overall" and "legalizing sibling marriage won't increase sibling sex overall" are likely to be inconsistent with each other, and that the first is implied by your wording; if gay sex increases, it should also increase "unprotected sex with multiple partners".
Are you kidding around at this point? On average, people who are married are going to have sex with a smaller number of distinct partners than people who aren’t married. Gay marriage, if it has a significant effect on gay sexual behavior at all, will clearly reduce the risk of HIV transmission.
With regard to sibling marriage, the issue isn't sex per se but children. Although I am not myself deeply opposed to sibling marriage, there are many couples who strongly prefer not to have children outside of marriage, so it is quite reasonable to assume that banning sibling marriage will reduce the number of children of siblings. In contrast, it would simply be laughable to suggest that any significant fraction of gay people who have unprotected sex reserve unprotected sex for marriage. If that were so, HIV would not be a problem in the gay community!
So, no, there is obviously no reasonable comparison between your two cases, as a few moments of thought would make clear.
>On average, people who are married are going to have sex with a smaller number of distinct partners than people who aren’t married. Thus, gay marriage, if it has a significant effect on gay sexual behavior at all, will clearly reduce the risk of HIV transmission.
You're neglecting the possibility of increased gay sex due to wider acceptance, which would affect even unmarried gays.
Can you make the argument for higher risk from legalizing incest in your own words, so we can see why it wouldn't apply here?
I'm neglecting it because it's not a realistic possibility. You can't just imagine any old wacky scenario and use it as the basis of your argument -- it has to be plausible.
Gay marriage would most likely have no significant effect on male-to-male HIV transmission rates. In contrast, it is quite obvious that legalizing sibling marriages could encourage siblings to have children, thus increasing the risk of babies born with genetic defects. That being said, it is not clear to me that this constitutes sufficient grounds for making sibling marriage illegal, and I am not strongly opposed to legalizing it.
If you seriously think that there are lots of gay men out there just waiting for gay marriage to be legalized so that they can have lots of unprotected sex, then you really need to increase the diversity of your social circle.