If we have selective service draft (meaning only a certain fraction of the eligible population gets called up, by lottery) and social conditions somewhat like OTL, but a military in which half the positions are filled by women...I'd guess it is a bigger military, with the male portion of it about equal in size to OTL, and with lots of "behind the lines" positions to be filled. Draftee women don't get sent to an infantry unit--there might be some volunteer all-women units that do everything, no matter how dirty and dangerous, but women unlike men can't be ordered into frontline combat. Women do the relatively safe support jobs.
Even for this to be allowed would require pretty radical shifts in society, back in the 1940s or '30s. It would be tricky to justify.
For one thing, I can't see society going so far as to demand that a woman who is pregnant or has just birthed an infant would be required to serve out a term as a draftee even so. Mind, a society which is determined she should serve out the time and provides the necessary supports to keep infants cared for during their mother's hitch would not be unthinkable and might be good in some ways, but it would be a major divergence from OTL. If the Army and other services aren't prepared to manage a nursery service for service babies, then getting knocked up is a golden ticket to a dishonorable discharge and a clean getaway from conscript service. Compelling draftee girls to use birth control would be difficult, might be medically dangerous (in 1940s-60s technological context anyway) and of course we'd expect a dozen major social power blocs to scream bloody murder on their behalf. The best single "protection" strategy (other than the abstinence from male contact the forces would no doubt be trying to enforce, but perhaps in vain) would be condoms for the men since it slows the spread of VD of various kinds as well, but it seems likely that a culture of draftee women would include some who are determined to have sex and if their legitimate options are nonexistent, will choose to take their chances with illegitimate ones, meaning they are in no position to insist their partners follow these rules.
An America which has undergone a really comprehensive sexual revolution decades before seems unlikely to me to be fighting a war like Vietnam on the terms we were at the time. Frankly if a shift in sexual mores goes so far as to make sexual activity purely a question of personal choice with no one else questioning who the partners are or other consequences, I would indeed expect a military that provides support for pregnancy, birth, and infant care during the term of service, with as little separation of mother and child as possible consistent with keeping the babies out of an actual war zone. So pregnancy is not a "get out of service free" card at all.
It seems to me to get a society like this we'd need a very left-wing, socialist to communist, revolution in the generations before. Note that even the Soviet Union did not draft women in peace time. During the Great Patriotic War of course hundreds of thousands of women took up arms in the riskiest front-line positions. Arguably the reversion of the USSR to universal male draft only was a step backward from their Communist ideals. Meanwhile Israel adopted universal training and service of both sexes, so the possibility is not as far-fetched as some might think at first. Note though that Israel too began very much under the domination of left-wing parties and initially enjoyed Soviet patronage and a lot of immigration from Soviet controlled territory--had the more religious right wing that has tended to dominate Israel recently had the upper hand from the beginning, it is quite possible women would not only not be required to serve but generically forbidden to, despite the heavy demand for soldiery Israel's situation places.
So it is by far easiest to see happening if the USA is transformed by a hard left movement of some kind, one that is quite happy to "subvert traditional family values."
My first impulse is to say such an America would not be getting into Vietnam type wars. But of course it is entirely possible it would anyway; that what appears to be a small brushfire conflict requiring just a handful of US forces to calm down turns into a spreading blaze that requires a far greater commitment to have any hope of "winning." And this happens in a sneaky sort of way, so that over years some Vietnam analog goes from "where's that?" to "yeah our brave kids are turning that place around" to "I hope to God my kid doesn't go there!"
I'd like to think a truly democratic American communism would deal with the issues on a more sane and frank basis, but I guess I'd be kidding myself. OTOH an authoritarian American Communism would not face the same sort of limits our half-baked democracy required, or one might say enabled, the people to hold the military/state establishment to. In an authoritarian Communist state, young men and women alike go in fresh-faced and with purest of hearts to do noble things, and the people of the intervened area love us, except for insurgents paid for by wicked foreign capitalist agents. If you meet a veteran of the fight who has rotated back Stateside and buy them drinks, you may hear a different story but unlike either a truly free Communist America or a bourgeois/capitalist one, their darker takes never get played up on national TV.
I just can't picture a society much like ours was in the mid-60s drafting women in the first place, and the range of choices we'd have of societies that could suggest to me that women would still be insulated from the front lines except by choice, and so the basic dynamic would still be the same.