I think you're reaching. Paul isn't talking about whether or not he is sexist, he's talking about whether YC is sexist. He's not saying HE has women FRIENDS he's saying YC is actually RUN in a large degree by women, one woman in particular - the co-founder of Y-Combinator Jessica Livingston. Not to minimize the other women involved, but it's just particularly relevant that the founder of YC could as much be said to be Jessica as Paul.
It's a clunky transition, but the transition goes from "I (PG) have a female co-founder" to "here is the makeup of YC as it applies to women" in the second sentence of the second paragraph.
He was accused both of being a sexist himself and of YC being sexist. I would say the "he is sexist" (for the sake of argument, let's assume that "sexist" means "particularly sexist" because it's hard to argue ANYONE is entirely innocent of being sexist) were pretty tenuous, but his attitude toward whether or not YC was biased toward men were pretty off.
This does a great job of explaining the latter, which I feel were a legitimate problem.
I'll give you this: "It's hard to argue I'm biased against female founders when I have a female cofounder myself" is a weak sentence. Yes, you having a female co-founder is strong evidence, but if you do sexist things then it doesn't make it any harder for me to argue you're sexist. The "it's hard to argue" part doesn't belong there, it belongs after he's not only explained who he chose to start YC with, but how YC and he have acted in relation to others in the industry, which stacks up extremely favorably.
That's a huge distinction.