She Did It On Purpose...
Look at the evidence....she sees a cute baby on the beach, all of a sudden decides she wants kids, he's not ready, she reluctantly agrees to respect his feelings, and BAM! Shortly after this she "accidently" gets pregnant. Yeah, riiiight. I realize birth control fails but very rarely unless the person is being careless. Plus it's pretty coincidental that even though they've been together for years, she only gets pregnant right after she decides she wants to. And the guy gets sucked into something he's obviously not ready for, all the while she's trying to trap him into marriage and a whole new lifestyle, which he's also not ready for, and then when things don't go exactly her way, she walks out, immaturely willing to make her baby grow up in a single parent home with a mother who can't even afford her own vehicle just because he wasn't acting thrilled about something that she, not he, even wanted in the first place. All because she saw a cute kid on a beach and selfishly decided on a whim that she wanted a family right away regardless of his feelings and readiness. Instead of waiting until they were both ready so he could propose on his own terms and their baby would have two *married* and *prepared* parents that wanted them, instead of just a selfish mother who wanted everything on her terms and a father who got trapped into even being in the situation.
If that were real life, and Hugh Grant had brains, he would have kicked her out as soon as she decieved him instead of bending to her terms. He could have still been a good dad even though it was forced on him, he just didn't have to be tied to the person who decieved him. This whole movie portrays him as being in the wrong just for not being ready for a child his girlfriend decieved him into having. My opinion is that, while he just wasn't ready, she just plain wasn't mature enough to have a child. He stepped up to the plate eventually and was there for her and his son, but odds are the relationship would never last in real life. You have to be a good partner before you can be a good parent (not that there aren't a lot of great parents out there who just haven't found the right partner or don't want one!), and there relationship will only last until something else comes along that she wants and he doesn't.
Before anyone rags on me, I am a female, I do eventually want kids, but I would never decieve my boyfriend just to achieve this. That's just not the way to bring an innocent baby into the world - by decieving their dad and trapping him into fatherhood. I'd never have kids with a man who didn't want and hope for them just as much as I did. Sadly, way too many babies are concieved through deliberate deception - whether to try trapping a man into marriage or otherwise - which no doubt is one of the causes of the many broken homes and deadbeat dads out there.