Actually, the purpose of the sex act is not soley procreation for a few lucky species - including us. Were this the case, we would only have sex when ovulation were occuring. The very fact that we don't (and so many other mammals do) shows that sex has a purpose beyond that of procreation.
I'd also duspute alijangra's point about lowering the limit, though I understand the reasoning. However, many deformaties of the fetus are not detectable before 20 weeks. These can include Spina bifida, Anencephaly, Hydrocephalus, Congenital heart disease, defects of the abdominal wall, kidney abnormalities, shortened bones, Diaphragmatic Hernia, Cleft lip and palate, Down's Syndrome and cerebral palsy.
To take one of the most emotive examples, check out this:
http://en.wikipedia.org/wiki/Anencephaly
I think most people wouldn't want to have the option of having an abortion removed if they discovered this apalling and horrific thing had happened. That's why, like it or not, the current limit is there; it's to do with when fetal anomalies become apparent.