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?
You make this statement as if the law were designed to discriminate against a group defined by an immutable characteristic, i.e., race or sex. These laws affect people based on their behavior, their choice of sexual activity. What you are arguing is that homosexuality is inherently more moral than polygamy, incest, adultery, etc., and is therefore deserving of different treatment than these other sexual activities. What Sen. Santorum is saying is that if you allow one of these activites as a constitutional "right", then you must allow all of them under the equal protection argument.
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.
Incest would only be covered under the age of consent laws if one of the participants was under the age of consent (which is as low as 14 in some states). Child molestation was not even addressed in this statement. And I am sure you will find someone who thinks they should be able to marry their cousin or sister... The question that will be raised will not be whether it is morally repugnant, but whether the government can prevent it once a constitutional "right" to engage in any sexual activity involving consenting adults is established. Such a "right" could not be restricted to homosexuality, once established.