I strongly suspect that today's large degree of lifestyle and relationship variations is due to the decline in the importance of monogamous marriage. There was a time when straight marriage (M+F) was everything both socially and economically. So much so that even gay people frequently got married to opposite sex partners. No doubt quite a lot of people *wanted* to behave the way they do today, but those modes of behavior, and types of relationships, were severely stigmatized. Yet we have now reached a point in the USA where a majority of women either live alone or with someone to whom they are not married.
So what is it that has reduced the importance of marriage and monogamous relationships with the opposite sex? I'm going to go out on a limb and assert that it's simply longevity. "Till death do us part" makes a lot more sense when you only expect to live to be 35, and if you want heirs to survive you, you'd better waste no time getting hitched, and start having lots of babies.
In 2009 almost 100% of sexual activity is purely recreational, not procreational. Plenty of people still get called "bastards," but the word has become an opprobrium related to one's character rather than one's parentage. Most people who have been married have also been divorced at least once. In developed countries, children are being born at a rate less than replacement value. (In fact as Asia also becomes wealthier, their birthrates are similarly declining, with the result that some demographers now project that the world's population will peak not later than 2050.) In the post-industrial age, family size has an inverse relationship to family wealth. Life is now long enough that people are no longer concerned about finding a life-long permanent partner, and may even consider it impractical. How can you expect to remain compatible with the same person for 50 or 60 years, after all?
In short, I think we are seeing behaviors and attitudes that were latent all along, but were merely suppressed by the requirements and expectations of society. Now that that is no longer the case, they are coming out to play. The whole world is coming out of the closet, in a way. Which suggests that it's not straight guys (or women) who are a dying breed, merely the predominance of their "standard" way of life. There are still just as many men out there who are turned on by female bodies (and who wouldn't be turned on by Punny's!) but the ways a woman finds them are in flux.
Cheers,
-DC