I understand exactly what you mean with the bolded, though it's true that you're not "supposed" to say that being in a good relationship or getting married makes you happier as a person. Contentment and happiness are two completely different things, though. Contentment is about acceptance. Happiness is about the positive feelings you have about what's going on in your life. If a woman has a source of love in her life that wasn't there before, I'd certainly hope she'd be happier for it!
I think that it's interesting that most of the pro-marriage and family messages in the media come from organizations that are run by men. That's not to say that they don't have anything of value to say, but I think that perhaps the traditionalist messages that are put out there are themselves skewed by the male perspective. A soon-to-be married friend was telling me that statistically, married men are always happier than married women. At a job I worked, two of the older men were telling me about marriage and one, a kind man, was saying how he hoped some young man would get his act together and pursue me properly. And without skipping a beat, a woman coworker said that when that day does come, she advises me to tell him "no." Another woman agreed. Both older, still married, but seriously considering divorce.
I think that the message that is unclear for younger women is whether the problem is with the institution, or whether women chose a mate poorly, or lacked self-knowledge in deciding to get married. I think that anyone dissuading women from getting married has to be clear as to exactly why they feel it's not such a great thing. Oftentimes, it seems like it's really about the individuals involved and not the marital arrangement itself.
I do think, though, that our society today makes it difficult to stick things out. Sometimes you have to be stuck somewhere to make the best of it. For the most part, there really aren't the social constraints on marriage that there were in times past, so people choose to drop it and pursue other options. So, I don't know if women are becoming wiser about the hardships in marriage so much as they simply have an easier time getting out of it. For this reason, I wonder if waiting till a bit older is going to prove to be the wisest course of action--waiting until you really do know yourself and what you want, and are perceptive about people to judge well, etc.