Birth control is the big one. You need to delay its invention and widespread adoption--maybe posit that it has some side effects that it doesn't have OTL--and everything else follows.
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Or go for a much more serious VD epidemic, maybe something horrifying like AIDS but that affects straight men and straight women more. I don't know much biology, though, so I don't know how deadly a disease like that can evolve to be. If its deadly and horrifying, you probably need it to have a long latency period so it can spread.
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or maybe some butterflies with the evolution of the Democratic Party in the 70s (remember that Ted Kennedy took an absolutist pro-life position back then). If the power shift from white ethnics to middle/upper class professionals, intellectuals, and activists doesn't happen, you'll probably end up with, over all, a somewhat more socially conservative set of laws, which will in turn influence behavior. It isn't going to be a dramatic difference, though.