Regardless of whatever choice made - once there is a baby -
WHATSOEVER decision one made will be WRONG. So why debate over an
issue where no decision is correct? 
People will consider and pick the choice with the
least degree of 'wrongness' (subjective and personal
preception).
Say your girlfriend is having your kid and you have to marry her
because she chose to keep it... unless you are being a bus-stard,
you would. By then again, in reality, how many short gun marriage
do actually last? Out of ten, as high as six/seven will fail.

You marry not because at the moment when you propose, you convince
and tell yourself that this lady in front of you would be the girl
you ever want to spend all your days with. You married because you
have no choice. 
Love must be bounded willingly. Liability is NOT an effect
of love. Humans are greatly influenced by moral, values
and senses, therefore you are bounded by responsibility, not by
cupid's arrow. Yes, you can put in your effort to make sure your
kid grows up straight, healthy and useful - but conscious effort
may not have equal result to the building of love. 
You may think I am deviating from whether to abort or give birth
question, however when the girl decide to give birth - you have to
marry her, unless you willing chose to be labelled as a jerk-ass
irresponsible creep. I am more concern over the aftermath.

P.S: Recently a friend just got married, shotgun case. Piangz, I
see liao... aiya people's business, forget it man. Anyway people,
wanna screw please use protection for god sake. Do it for the sake
your own karma, if not for your gf. 
Cheers