If a man gets a woman pregnant, must they marry in?

What does islam have to say on this subject?

Islam is very strict about/against women having sex outside of
marriage, premarital or with any man other than the husband.  This is
not an attitude that started with Islam, it was a tenet of society in
the Middle East before then, recall the Bible story about the stoning
of the adultress (Jesus:  "Let he who is without sin cast the first
stone.")

http://en.wikipedia.org/wiki/Stoning

For some peoples from the Middle East with conservative traditionals,
premarital sex without pregnancy is a dishonor to the family that can
only be "repaired" by members of the family killing the young woman. 
It happens;  here in Germany there was recently a case of brothers'
murdering their sister.
As said, this strict attitude is not original with Islam, but the
religion incorporated it in its tenets, "exporting" the principle to
areas of the world that may not have had such a strict attitude on the
matter.
Perhaps a marriage can correct the offense in those areas or among
less traditionally minded Muslims, those with more western attitudes,
but you asked about Islam.

I wold say Islam is very strict about sex outside the marriage
institution however if such mistake did accre the way to rectify in a
good way thogh not a rle is to marry the lady and take care of all her
needs etc.

In the Fiqh Jaafria (Shiates) It is possible to marry temprarily i.e.
express lining with each other and here again women's situation is
very well though which is that if with this relationship women gets
prgnant the man has to pay the des of the baby ntill the baby is grown
up or if the lady has re-married and has no need of the provision from
the father of her child.

Islam has fully taken care of women in all aspects so saying that
Islam prohibts women having sex is wrong it is for both men & women
that they should marry rather then have sex and leave woman in troble.