TS, you have a girlfriend. So it kinda revokes your right to have ONS. And the older woman was not consenting. If you had to tell her it was just a fling and that you won't leave your gf for her, that means she actually thought you would do that. Plus if she really was consenting and considered and abortion, she wouldn't have to contact you and tell you about it. she would just go and have an abortion done.
Sorry to be superstitious, but if you're chinese, it is believed that aborting a child is sinful, and after the abortion, it will become a naughty ghost, following you and making your life or your gf's life miserable.
on the reality side, abortion is bad for the older woman. many many reasons have been mentioned. and the really ironic thing is, doing the abortion relieves all risk on you but all the risk falls on her.
and you have responsibilities. Firstly to the woman and child. You humped her, baby arrived BECAUSE you humped her. Baby is part of YOU. When you kill the baby you are killing part of yourself. You should have realized the possibility of this happening when you ONS with her. She's in this shit because of you. don't you think you should be responsible and do the right thing?
secondly, to your gf. you have to tell your gf you humped someone other than her. why? well, for starters, it is widely known that when a girl has sex with a perso with multiple partners before, she is also figuratively speaking having sex with all your other partners. risk of disease and such. she's got a right to know.
secondly, you're her bf and she trust in you to be truthful and faithful. you betrayed her trust. she deserves to know.
thirdly, you can't have a gf and still hump other females. a relationship is a devotion to each other. would you be ok if your gf is being humped someone else right now? if yes, then you're an idiot and obviously don't need a gf. if no, you're a selfish *******. why can you do something she can't.
be a man. do the right thing. take up responsibilities that were caused by your mistakes. wipe your own arse after you shit. after all, everyone's (that would be the woman, your baby and your gf) in trouble because of you.
....
i have nothing to say in my defence...