If even the Lawful Good alignment doesn't prohibit sex outside of marriage, what's the point of characterizing sexual mores with alignment?
I think it reflects more the author's sexual preferences (free sex is good, duh) than any real reflection on the nature of alignment and sex.
And let me anticipate an objection, that chastity might be promoted by a particular religion that holds to LG. Ok, fine. We might assume that the religion will state that "sex outside of marriage is evil".
"Oh yeah", says the radical priest, "I cast detect evil on those fornicators over there and i didn't get any evil".
God: "Well, its disorderly, then"
"Nope, detect chaos isn't active either"
God: "umm. you shut up now"
An excellent case could be made that the lawful part of lawful good would particularly disallow fornication as being harmfull to society and institutions like marriage. But since Anthony "Vile Darkness" Valterra is coming from his Chaotic Neutral perspective on sex, he only addresses the LG alignement from the perpective of a "personal alignement", rather than seeing the Paladin as part of a group.
Fascinating, really.