I want to be patient with him because it is clear that he appreciates the basic question - there is obviously something different between the broad acceptance of sexual activity and a similar acceptance of abortion. Whatever could it be? Um...now let me think. Could it be because the latter involves a dead human being?
A sociologist cannot draw that conclusion. The furthest he can go is to conclude that there is a perception that the killing of a human being is involved. From there it is easy to fall back into the comfort of the position that it's only a matter of perception, and that the perception is likely to be erroneous.
Well, it isn't. It's a matter of stark appreciation of biological fact. That thing isn't undifferentiated tissue, it is a person. I will quote Christopher Hitchens on the issue: if it isn't human, what is it? And if it isn't alive, what is it?
It is, to be sure, a perfectly legitimate sociological question as to why so many of us think that the death of a human being is involved. "Because it is" is a terrifying answer. But it does lead to an equally valid question that I challenge him to ask: why so many think that the death of a human being is not involved.
Easily answered.
Of course it is biologically human. Of course it is alive. Of course it is a human life.
Those are scientific questions with obvious answers.
The question is not whether the fetus is a human life, it is whether it is (or should be) a legal person with the rights of other persons.
This is not a question that can be answered by science, as it is moral, ethical and legal in nature. In fact, it is at root a theological question.
It is entirely logical, though deadly wrong IMO, to believe a fetus prior to the moment of birth is not a person and has therefore no legal rights. Or, more accurately, choosing the moment of birth as the point where such rights are acquired is neither more nor less logical than any other random point. This creates logical problems for proponents of abortion, as we can see from the recent arguments that "post birth abortion" should also be allowed.
Each human life is a continuum from conception to death, whether that death occurs 3 months or 100 years after conception. There is and can be no point on that continuum where it becomes logical to say that life should acquire (or lose) the "rights" of a "person" under the law.
I think the non-religious case against extra-marital sex is less persuasive. That's probably why more people think it's OK. Even though a large percentage claim to be Christian, they are Christian in name only. They ignore the clear teachings of their religion, so the only thing that would make them oppose sex outside of marriage would be the non-spiritual repercussions.
I suppose one could argue that sex before marriage increases the spread of disease or causes out-of-wedlock pregnancies, both being negative results from a purely secular/non-religious point of view, but one can mitigate those results with birth control and disease prevention. If one is willing to ignore the spiritual ramifications of not doing God's will, then extra marital sex doesn't seem as bad as abortion.