If I said, "If you allow (gay) consensual behavior based on privacy then you allow eatiing, sleaping, showering, or watching T.V." would people howl? Of cours not, because there is nothing immoral about any of those things.
Santorum picked four well recognized immoral acts. The homosexual community, which seems to not understand the concept of immorality, got mad to have their act lumped in with those others. Why? Show me one moral code practiced anywhere in the world that doesn't make homosexuality as immoral as bigamy, polygamy, adultery, and incest? There isn't one, which means that it was perfectly reasonable to lump homosexual behavior in with the other four.
The homosexuals are ticked off because it reminds everyone that homosexuality is immoral.
Shalom.