Honestly, that's quite irrelevant.
Sex does spread diseases. Many diseases that are easy to treat with modern science existed 3000 years ago. But they didn't have penecillin or tetracycline, so syphilis or gonorrhea was as fatal and contagious as HIV.
Strict rules about sex made sense in the primitive world.
Tom
While you're right, I'm not confident that's the reason such things were prohibited in the Torah. Monogamous, STI-negative gays aren't any more likely to catch a disease from gay sex than monogamous straight people. Nor would catching a disease from sex morally justify executing that person. Lepers were at least segregated, not killed.
Gay sex is described as as an "abomination," in the classic English translation, which as I understand it is a Hebrew term used to describe a wide variety of offenses, including cross-dressing, idolatry, and cheating in business. So the term seems to carry more of a moral judgment based on some standard besides hygiene.