Two homosexuals get married. They have unprotected sex. (As married couples are wont to do.) One contracts HIV from the other.

A father and daughter get married. They have unprotected sex. (As married couples are wont to do.) One contracts HIV from the other, and gets pregnant, and has a genetically-unstable child.

Did I not already explain the difference several posts ago?

TL;DR
The consequences of one relationship are even more serious than the consequences of the other relationship.