Somehow Margie is a pervert for being attracted to a guy who is five years younger than her (at this point she is thought to be 23... she's really 22). But somehow, Bill how married a 16 yr old (thinking she was 18), is NOT a perv?
In many ways, bill is the exact same as some of those man on the compound
I think their reasoning was because Margie was Ben's "mom" it was more perverted in bills eyes than him marrying someone young enough to be his daughter.
But back on the OP's original point: a device that repeatedly pops up throughout the show is clash between Bill's "principles" and his actual behavior. An obvious example, that characters explicitly point to, is his strong feelings about having been thrown out of the compound as a child and his expulsion of Ben (even if it's temporary). Bill is repeatedly revealed to be a hypocrite.
I agree he is a hypocrite in many areas but I don't think he expelled or exiled Ben. I think that was really and truly a misunderstanding. Bill was exiled as a teen, homeless and cut off from his family while Ben was sent to spend some time at his sister's place. Sarah was still very much a member of the family even though she wasn't living at home anymore. Bill should have been clear with his intentions from the start, though.