So this will look like a really weird and hilariously petty argument to anyone who hasn't been exposed to a lot of conservative Catholicism, so here's some context: the official teaching of the Catholic Church is that the "marital act" (PIV sex) must be both "unitive", contributing to the unity of the couple spiritually and emotionally, and "procreative", meaning it's capable of producing new life. These definitions actually get really fuzzy because they reject IVF because it's not unitive and reject anal sex with ejaculation because it's not procreative, but still allow infertile couples and pregnant women to have sex... but that's beside the point right now.

What's important now is that /u/kmo_300 posted an excerpt from a Catholic catechism by an amateur theologian (he only has a B.A. in theology and his views aren't backed by any bishops or official hierarchy), and it vehemently argues against any kind of oral or anal stimulation during sex. This sets off some of the very slightly less right-wing Catholics in the sub, so let's get started.

The full thread

Someone calls out the amateur theologian as the source

Some arguing on whether the woman has the right to an orgasm

One side there is basically saying that it's selfish to seek pleasure in sex outside of what is required to get semen into a vagina (here)

Here we get a breakdown of established theological opinion on the subject, with one saint being listed as saying that oral sex is sodomy

...and an angry response questioning the parent's credentials

People keep arguing for awhile about celibate men's opinions on blowjobs...

There are a few more top-level comments with counter-arguments but you can find them yourselves.

And now you've seen some of the weirder side of the wacky wonderful world of conservative Catholicism!