The boyfriend is their fellow classmate, but he's too big for his age and has a shaved/bald head and is the quiet type, so he looks much older than they are, like he was either a professional athlete or a gangster/mobster.
The heroine is head-over-heels-in-love with him (like in the manga/anime My First Love) but her three BFFs are paranoid about the guy and think that he's just using her for sex (they do it a lot).
So, they keep on trying to come between them until their friend gets pregnant and her boyfriend proves that he really does love her after all by being happy that he's going to be a father and is of course willing to marry his girlfriend.
The BFFs then get stricken by their conscience and now wonder if they've in fact already become "rotten" because they were actually secretly envious of her all this time.
I saw this on another site a few months ago, and it was translated already, but it had a really generic title like "Average/General/Normal/Traditional/Usual Physical/Sexual Attraction/Interaction/Relationship" or something, which is why I can't remember the exact title and thus I can't seem to find it again.