He was talking about a whole range of sexual practices that could fall under the "privacy" exception, if the USSC decided to use that reasoning. The "clarification" was to raise the ire of a specific, very political group. If, as you suggest, he was obviously talking about gay sex, then the "clarification" was unnecessary. There was an agenda at play here by the reporter, and she got called for it.
I've been thinking about the consequences of the Supreme Court's eventual decision, not about Santorum's particular pickle. The transcript is enlightening. But I think his reasoning is probably not accurate. The Supreme Court can decide that the Texas law is now designed to limit the practices of one particular group (homosexuals), and therefore is an unconstitutional limitation of their rights, yet at the same time maintain that other practices are morally unacceptable and deeply repugnant to society, and therefore the State still can have laws against them. Incest and child molestation would be covered, both essentially by "age of consent" considerations, and I doubt that anyone would question the laws against marrying one's siblings. (And one wonders if Senator Santorum would have as much of a problem with that; it would constitute a heterosexual marriage relationship, of course, likely with the 'traditional' activities in the bedroom, too). Is it the particular act or the relationship that most concerns him?