I think the pregnency issue has a great deal to do with it.
Also, as my father said: "I know it's not fair, but it's the way it is. My son fucks with every one he meets male and female and folks tell me 'That son of yours is some stud', my daughter sleeps with every guy she meets and she's a slut and behind my back tell each other 'It's too bad his daughter can't keep her panties on, he must be so humiliated' and frankly I am."
I also think that women traditionally were the home makers and raised childen (the result of their fucking) they saw faithfulness and monogamy is a very different light then men did and still do.
Traditionally the clothes women wore were designed to make having sex difficult but the clothes than men wore were designed to make having sex quickly and easily.
In the 1950s, 60s, 70s and 80s there were no established role models for gay couples who wanted to have a permanent relationship. They felt they could be as creative about it and what worked for them was the way they should do it. (Also during that time a man's introduction to being gay was nearly always through 'man sex' and other through a sexual experiences, today I know a number of college age guys who say "I know I'm gay, but I haven't acted one it yet." When I went to college it was the different: "I sure have fun jacking off with the guys but I know I'm not gay." Some eventually realized they were others married and raised families, they may have lived in terrible closets, but they may have found happiness in the security of a home and family.
Today monogamy is so popular among young gay couples. It's really shocking for me to hear a young man say, "Oh I couldn't think of sleeping with anyone other than my husband/boyfriend/whatever"
Shortly after I met the man with whom I've had a very long term relationship, we both confessed we knew we had "met the one" (he was married to a woman and had to unravel that before we could live together)--for that and because we both truly loved having sex the first rule we imposed on our relationship was "Either of us could fuck anyone we met once, the second time it had to be a three way and from there on it was free form." It is the only rule we've ever had. We both have had pretty intense relationships with other guys and girls but I can honestly say we have never thought of dissolving our union.
I know very few women who understand our attitude toward promiscuity, but it's given us a fabulous life together. Since the early 70s we've traveled the globe together, tricked with locals from other countries, visited bath houses and whore houses and gone our separate ways.
We both understand sex is for fun. There is so much more bonding our relationship together though our sex has always been the best I've had.
When I was actively bisexual before I met him I always maintained "I don't understand why people want their partners to be faithful to them. I want to world to know my wife/boyfriend/spouse/lover is the best fuck around, and further more I want them to enjoy it That would really make me proud." I mean who'd want someone without experience and who has nothing to compare their sex with.
I know that because we both had a history of promiscuity before we met we understood the others desire (and it was a desire not a need) to remain sexually active, that was one of the things that we were attracted to in each other.
But then, there was no one to tell these attitudes were wrong. Relationships were about being together. We both thought relationships were about having fun. I know my parents was (I don't know if they slept around, I wouldn't fall apart if I learned they did, but they had such a beautiful life together and shared so many wonderful things and they had fun together, always they had fun.) Why should anyone care what you do for an hour or two with another human being, but society and our culture tell us that everyone does, that God does, that it is irresponsible behavior.
I think our lives were more creative and henceforth better before colleges offered courses in "Gay Culture". Gay History is important but "Gay Culture" has always been based on the easy availability of sex. This was true of the Castro in San Francisco, of Greenwich Village in New York, it also is true of Berlin and Prague.
Let the good times roll!