The MC is a guy I think is autistic and the neighbor girl watches him. She takes him to a party I think where she gets drugged and they get him drunk. he ends up getting a group of girls pregnant. later he sees one of the girls get hit by a car and takes his child home.
That is about all I can recall.
Thanks for any help.
I think the below story is what you are looking for:
https://storiesonline.net/s/48122/judgements
I don't believe this is the story the OP was referring to. IIRC, this story takes places almost entirely within the college years of the MC and the BF pretty much ignores the fact that the MC has changed from the young boy she first befriended throughout most of the story. The MC has been in love with the BF but afraid to say anything.
However, I believe the story the OP is referencing starts somewhere around when the MC is about 14 or 15...and he is asked to escort the neighbor girl who has been a close friend to a dance or something with a few of her friends. IIRC, the neighbor girl had a recent injury she was taking meds for. All the girls and the MC ended up at one of their houses. The girls (including the neighbor girl) were all a year or 2 older than the MC and are curious about sex so decide to use the MC to learn more. All the girls have been drinking and the neighbor girl also taking her meds. I believe something happens with the interaction of the meds and alcohol stopped her breathing for a time, and by the time someone realized it and got help, she ended up in a coma for a time and came out of it with a changed personality...and pregnant. IIRC, the MC said he would marry her and take care of the child, and he did so...eventually though she divorced him and made his life hell (the changed personality, drugs, alcohol?? cant really remember reason why). His daughter was similar to him mentally (limited social/emotional interactions) and one time while picking up or dropping off his daughter during one of his scheduled visitation periods, they both see the girl's mother get hit by a car. They just look at each other, turn around, and go get ice cream on the way back to his place. The story takes place over a period of at least 10-12 years (possibly longer) and the initial group of girls over the years pretty much used the MC as a Dial-A-Dick. They all ended up caring for him, but realized that he really couldn't connect the same way with them. Several ended up pregnant by him, but I can't remember if he realized he was the father or not.
Unfortunately, although I looked, I could not identify the story name and author. I only read it once a few years ago.