that STD's are for the most part spread through promiscuity. Polygamy will not spread STDs unless combined with promiscuity.
If the whole issue here is a comparison of homosexuality to polygamy, then it is a closed issue. It is patently ridiculous that homosexuality and gay marriage should be legal in a country which criminalizes Islamic polygamy. And STD's is only one reason.

Also, if anything thinks that laws against polygamy are not enforced against Muslims in the U.S., they are very mistaken. If they know you have more than one wife, and if you have children, they are likely to take them from you and put them into foster care.

Even several women who have converted to Islam in the U.S. have had their children taken and put into foster care for simply that reason, especially in redneck states like Texas, etc.

It is serious business.