Once upon a time (well, all right, in the days of the so-called Moral Majority), Evangelical Christians and various other social conservatives opposed both same-sex marriage and abortion rights. However, they were more concerned with the latter, as Roe v Wade had become the law of the land (which they were trying to repeal) and there wasn't as much of a movement for same-sex marriage as there has been in the past few years. Also, in those days, even most people who favored legal abortions opposed same-sex unions, whether on religious or other grounds. So there really wasn't much reason for Jerry Falwell and his friends to get worked up over Adam marrying Steve.
The fact that Evangelicals were against abortion and same-sex unions was really about the only thing those two issues have in common. And, oh, yeah, they both involve sex, which is probably the reason they got Jerry and his friends all hot and bothered.
But more recent polls show that younger people--even the children of those fundamentalist Christians who helped Ronald Reagan and scores of state and local officials get elected--support, or are willing to entertain the idea of allowing, same-sex marriage. And many of those same young people want to overturn Roe v Wade, as their parents did or do.
Part of the reason is simply that younger Evangelicals have grown up in a different world from what their parents knew in their youth. But, more to the point, I think, is that abortion and same-sex marriages aren't just apples and oranges, or even apples and onions. They are, simply, profoundly different issues.
First of all, for all of the controversy it has created, abortion is still mainly a private issue. The girls and women who end their pregnancies do so, for the most part, alone. Their boyfriends, husbands and families may be involved in the decision, but no one else is--save, perhaps, for the abortion provider, if the procedure should go wrong and he or she faces a malpractice suit. Most girls or women who go through the procedure get on with their lives because, well, for most of them, that's the point of getting an abortion. The few exceptions are those who have to end their pregnancies for medical reasons.
On the other hand, getting married is the most public action most people ever take. The union of two people affects themselves, their families and many other people in their community, mostly in positive ways. In contrast, almost nobody is happy about an abortion. The woman terminating her pregnancy as well as other people in her life might feel relief, or at least that the least bad choice is being made. But almost no one who has ended her pregnancy will tell you that it's a cause for celebration.
Also, there are many legal ramifications to marriage. They include tax benefits, inheritance rights, health care, insurance, hospital visitation rights and custody over children and, in a few cases, other relatives.
Those consequences (both in the positive and negative sense of the word) last as long as the marriage does. Most people, when they get married, want their unions to last for the rest of their lives. And, if they don't, ending their marriages certainly has little, if anything, in common with ending a pregnancy.
Finally, let's just say that the kinds of people who want to enter into same-sex unions aren't, generally speaking, the same people who get abortions. If same-sex couples want to have children, and they don't want to adopt, they have to find surrogates: One to impregnate one member of a lesbian couple, or one to carry a child for a gay male couple. Somehow, I don't imagine that very many people in those circumstances think about having abortions! In fact, about the only way abortion might intersect with the life of anyone in, or who wants to enter, a same-sex union is if he or she is bisexual and has a heterosexual relationship outside the marriage, or has had such a relationship before getting married.
All of those things being said, I will reiterate a position Hillary Clinton articulated when she was First Lady: Abortion should be safe, legal and rare. And, as I've mentioned in other posts, I support same-sex marriage because it's the best we can do for same-sex couples until the state and churches lose whatever power they have to determine whether or not people are married, and until there are no longer any tax or other benefits to marriage. That is why I support both the right to same-sex marriage and abortion, and my support for one is not tied to the other.