That wasn't really my whole argument. Basically it's this:
1. Santorum never said he thinks sodomy should be illegal. In a question specifically in regard to same-sex marriage, he appealed to natural law to justify his opposition to same-sex marriage. He did not go further and suggest that current law w.r.t. sodomy should comport with natural law (as he understands it).
2. Even if Santorum did think sodomy should be illegal, sodomy need not include oral sex. In fact, biblically speaking, it doesn't. Various civil laws have, in the past, included "deviant" acts (such as oral sex) in their laws banning sodomy, but sodomy needn't be understood to include those acts.
3. Even if Santorum did think sodomy should be illegal, and did think that sodomy includes oral sex, as president he would not be in a position to act on those beliefs without a Congress that also shares them. Good luck with that.
4. Even if Santorum thinks sodomy should be illegal, and thinks oral sex qualifies as sodomy, it's possible that he holds federalist views that would preclude him even attempting to restrict sodomy (and/or oral sex) at the federal level.