I would say, rather than being sexist, it's just being honest about its target audience, which could potentially have the effect of alienating people who aren't its target audience, but it would only, IMHO be sexist if men weren't allowed.
The way I see it, there are many, many forums on the internet; I used to be a member of a pet forum - it was predominantly for pet owners, but non-pet owners were allowed to come in, ask questions, participate etc. It was just aimed more toward people with pets.
I first heard of MN through my best friend when she was first TTC. I signed up at some point because it looked like an interesting place, but I lurked for such a long time because I felt like I didn't belong, even though I had views on many of the non-parenting topics, mainly because I'm not a mum. I've only started posting this year because I'm starting to plan a pregnancy (using IUI with donor sperm, so not your typical potential mother) and I knew when the time came that I had questions, here's the right place to be.
So yeah - that's my thinking...
That said, my inner devil's advocate is asking 'would it be ok to have a WhiteNet website aimed at people of a specific skin colour but open to all people to join it? My immediate response is that no, that wouldn't be ok, but I can't pinpoint how/why that's different to MumsNet or Men's Health, or Women's Choice...