This was something I was unsure of, but I'm glad Ed Feser clarified. Or maybe some Roman Catholics disagree. I'll have more to say on the entirety of Feser's post, which replies to Andrew Sullivan's discussion of his work in The Conservative Soul, later. But for now, I note, I was unsure whether doctrinaire Catholicism ever permitted oral sex between married couples. According to Feser, it does.
For example, Sullivan describes a “Catholic married couple who live their lives according to natural law in every respect” as one who “never engage in any sexual act that does not result in the penis depositing semen in a vagina” (p. 84). If what he means by this is that the Catholic Church or natural law theory forbids acts like fellatio and cunnilingus even between married people, he is mistaken. What is forbidden is taking fellatio to the point of orgasm, or taking cunnilingus to orgasm outside the overall context of a completed act of intercourse; it is not necessarily forbidden to indulge in them as foreplay to an act of intercourse that results in ejaculation within the vagina. Perhaps Sullivan realizes this, but if so he should have expressed himself more clearly, since he is bound to give unwary readers the impression that natural law and Catholic teaching are more restrictive than they really are.
Is there an official Catholic document on this? I would think that fellatio would still be forbidden. It really depends on the man, and his "trigger." But wouldn't allowing a married couple to engage in fellatio greatly "risk" seed spilling outside a woman's vagina? Perhaps fellatio to the point of erection would be okay. But fellating an erect penis? I would think a Thomistic Catholic would say: "No way."