Well, he also seems to be oblivious to the impropriety of a superior officer shagging one of her lieutenants, so I'd say it probably wasn't about Riley's morality. :)

Although, to be fair, they didn't actually have a relationship until the spring semester, at which point, Buffy wasn't in Walsh's class anymore. It may have been inappropriate for Riley to make overtures while he was her TA, but it's not the same as if they were dating or sleeping together.