It isn't so fuzzy for me on that point. It has something to do with what we can "create" and what we can't. A sperm cell is no more sacred than a blood cell. Indeed there are other types of cells that we can manipulate into becoming reproductive cells. This isn't to say that we can create life, but rather that disposal of either eggs or sperm cannot really be argued against with the same fervor as a viable fetus.
The late trimester thing is pretty clear cut to me, but on several grounds. Yes the infant is viable and could lead a happy life. Just as important though, is that I view many abortions in the realm of shunning responsibility. Intercourse allows for the possibility of pregnancy. If you're responsible enough to do it, then you're accepting responsibility for the outcome.
If you are then indecisive enough to take 6 months to make the decision to terminate, your choice is made for you on the side of life. 3 more months is very little time and carrying the baby full term is a nice deterrent from getting in the situation again.