I’d be careful about using this as an argument against homosexuality per se. The reason homosexuals get more STDs is because more of them are sexually promiscuous and lead idiotic, hygiene-be-damned lives (as mandated by their sorry “culture”), not because homosexual acts themselves are any more “dirty” than heterosexual acts. If you look at gay magazines and “pride” events (if you have the stomach to do so), it’s always about sex, sex, sex—there is never an interest in caution and moderation (because that’s for evil, sexually repressed Christians).
Sleeping with the wrong gender doesn’t cause disease. Having sex with anyone who asks and thinking you’re too magical to face any negative consequences will do the trick.
The real argument is because the Bible, as part of its moral law, shows that He created male and female uniquely compatible and complementary, and only joined them in marriage, which is the only context in which sexual relations are sanctified.
The deleterious effects of promiscuous sex of both kinds is an argument in favor of the wisdom God’s laws.
Romans 1 speaks of an afflictive penalty* for male homoeroticism, and male homosex is medically indicated to more easily spread infectious diseases, due to details between the MSM sex and that which a women.
While this aspect could be an argument for homosexual marriage, God’s complementary joining of male and female goes beyond mere physical considerations and His design for such, while the only studies done of homosexual marriage show that they most typically redefine it in terms of rejecting monogamy as well.
*”And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet.” Rm. 1:27