This is indeed the tradeoff and downside. Overall I do not think it's worth dealing with the male/female headaches that come in a mixed forum, however, this is by far the strongest selling point in favor for a mixed forum.
That said, I would like to point out that everyone's profile is also a "mini-forum". People can talk to you publicly, if they click on your name. So if you want to talk to guys in your profile you still can, and it will still be a public space: your public space.
So if you are married, for instance, and do not want PMs from other men, then you can just tell them to post in your profile and post so others can chime in.
Likewise, if you have a question or want to discuss something with a guy in a public space, just click on his name and post a question or comment. Everyone can see it, it's a nice way of organizing things actually and I hope people use it more.
The downsides to keeping the public forums sex-separate are easily circumnavigated, while the upsides keep discussions focused and pleasurable for everyone to read.