From what I've read, later documents suggested that he was internally very conflicted on the issue of abortion.
Hunter S. Thompson laments McGovern being smeared as the "triple-A" candidate in Fear and Loathing on the Campaign Trail '72. As I understand it from that book, McG was legitimately opposed to it on moral grounds, but did not think government had the authority to ban it. I myself think that McGovern would probably have made some moves to limit access to it on the Federal level, and though I'd have opposed that, it ironically would have been more than the pro-life position has ever gotten out of the Republican Party.
Of course, the GOP doesn't actually want to overturn Roe v. Wade, because it doesn't want to lose those pro-life single issue voters. Better to keep the issue alive. Same thing with Obama and the gays; Support them rhetorically, and toss them a bone here and there to keep them satisfied to keep the money and votes coming in.
