OK, so, in the style of Saturday Night Live's "REALLY?" skit:
REALLY? Are you REALLY saying that marriage is EVER sanctioning self destructive behavior? Is that REALLY upholding your family values? REALLY?
REALLY? So, do you REALLY think that unmarried men are encouraged to not spread their obviously gotten-out-of-wedlock-sex STDs by remaining unmarried? REALLY?
REALLY? So if they marry a woman, it's OK to spread that disease to her? REALLY????
The complete and utter fail of this line of argument should be apparent to just about anyone. Seriously, if you take the stance that marriage is all about commitment and partnership, as it's traditionally viewed, you'd think that marriage is a SOLUTION to the increased rates of STDs. Sure, you can argue that there are men who don't believe in monogamy in marriage (shock: there are women who believe that too), but if you're advocating traditional values, then you should assume that your marriage ideal is that of traditional commitment.
So, showing how bad your argument is, a proof:
Assumption: Traditional Marriage is a committed partnership between One Man and One Woman.
Assumption: Homosexual men have higher instances of these STDs.
Assumption: Women, as a whole, have lower instances of these STDs.
Assumption: Men marrying men increases the risk of STDs to the general populace, which is a "secondhand danger".
Thus: Men should not marry other men.
Thus: Men should marry only women.
Thus: Homosexual men, who have higher instances of STDs, should marry women, who have lower instances of STDs.
Thus: Women become more infected with these STDs, as a result of marrying infected men.
Thus: The general populace becomes more infected as a result of requiring "Traditional Marriage".
Thus: The general populace has more "secondhand danger" when "Traditional Marriage" is required.
So, in fact, this argument actually leads to the opposite of your claim. Perhaps we should use this on the pro-gay marriage side of the fence! Gay marriage saves lives!!!!