That struck a chord with my reading too, that Marianne was defending her actions by there being a good feeling. It is very much something that can equate to modern times, too, good call. It's pretty flippant. Yet she blushes, which can mean different things in JA's novels.

Seems though, from what we know from Elinor, that her sister's moral compass was completely off centre, as she didn't do any of the cautionary things that the former seems to have expected her to know.