From a good essay by Digby back in 2005:

The war provided two very distinct tribal pathways to manhood. One was to join "the revolution" which included the perk of having equally revolutionary women at their sides, freely joining in sexual as well as political adventure as part of the broader cultural revolution. (The 60's leftist got laid. A lot.) And he was also deeply engaged in the major issue of his age, the war in Vietnam, in a way that was not, at the time, seen as cowardly, but rather quite threatening. His masculine image encompassed both sides of the male archetypal coin --- he was both virile and heroic.

The other pathway to prove your manhood was to test your physical courage in battle. There was an actual bloody fight going on in Vietnam, after all. Plenty of young men volunteered and plenty more were drafted. And despite the fact that it may be illogical on some level to say that if you support a war you must fight it, certainly if your self-image is that of a warrior, tradition requires that you put yourself in the line of fire to prove your courage if the opportunity presents itself. You simply cannot be a warrior if you are not willing to fight. This, I think, is deeply understood by people at a primitive level and all cultures have some version of it deeply embedded in the DNA. It's not just the willingness to die it also involves the willingness to kill. Men who went to Vietnam and faced their fears of killing and dying, whether voluntarily or involuntarily, put themselves to this test.

And then there were the chickenhawks. They were neither part of the revolution nor did they take the obvious step of volunteering to fight the war they supported. In fact, due to the draft, they allowed others to fight and die in their place despite the fact that they believed heartily that the best response to communism was to aggressively fight it "over there" so we wouldn't have to fight it here. These were empty boys, unwilling to put themselves on the line at the moment of truth, yet they held the masculine virtues as the highest form of human experience and have portrayed themselves ever since as tough, uncompromising manly men while portraying liberals as weak and effeminate. (Bill Clinton was able to thwart this image because of his reputation as a womanizer. You simply couldn't say he was effeminate.)