Somehow happiness, in the sense of pleasure, is now an expectation. Pleasures are so common in our wealthy land that it seems they're taken for granted. And as Screwtape notes, "love" is warrant for any sexual conduct and any broken promises. If you remonstrate you're accused of opposing love. Of course no society regulates love, but all regulate sex (even ours still regulates some). But that's an inconvenient observation.
Part of the shift lies in the connotations of words. Take the word "heterosexual." It is generic--there's no connotation of monogamy. Instead the sense is of willingness to have sex with any receptive woman. Male point of view here, reflect as needed "Bisexual" is similar, but the connotation of action pretty much guarantees infidelity. If such terms are the only ones in the discourse, there's a bias against considering fidelity.
I propose a modification of the weaker Sapir-Whorf hypothesis.(*) Language shapes our thinking, but we spontaneously modify the language to suit our needs, so the shaping need not be overwhelming or permanent--unless someone is able to keep pushing the desired usages.
If I and my friends compose a nuanced phrasing that defines something precisely, we can communicate accurately with each other, but if this usage isn't shared by the popular media we're immersed in, it won't spread. As a trivial example I give you Blackacre, about which many precise things are said that are unintelligible to non-lawyers--though they're not hard to understand. True, nobody pushes the standard fuzzy meanings about ownership, but nobody tries very hard to teach them either and they're not matters of our usual daily round.
Less benignly, think of the campaign to replace the word "gambling" and its connotations of risk with the word "gaming" and its connotations of innocent fun. In Wisconsin, at least, the latter seems to have pretty much displaced the former, and one finds it harder to find people who object to gambling in principle. Whether there are fewer who object to gambling as a form of fleecing the poor I can't tell; I wasn't paying a lot of attention years ago.
(*) Should we call it the Sapir-Whorf-Confucius Hypothesis? "If names be not correct, language is not in accordance with the truth of things. If language be not in accordance with the truth of things, affairs cannot be carried on to success."