Let's face it the big one, the incurable one, is HIV. The one you really need to worry about. Wiki gives you some interesting statistics about you're percentage chance of contracting it per act with someone who has it. You can't really do the math without knowing the percentage of WG's who have it and I don't know that (I doubt any statistics you might find would be at all accurate or reliable).
But I imagine you're calculation if you could do it would tell you the risks are actually very small PER ACT. But the point is you're not going to do it just once anyway ! Yes, a nice clear test results from a WG saying she is HIV free is great...until you realize HIV takes up to a month or more to show on a test. So she has a test today and shows it to you next week and there's at least five weeks she could have contracted it and you or she wouldn't know. Certificates mean nothing, except statistically less chance she has it. Again, it's all about the risk you're willing to take.
I'm very new to all this but I have made the decision that I will not be having sex with WG's because I don't have sex with a condom (yes I know, very stupid, but I have never got on with them and it would mean I never have sex so I'm prepared to take the risk, but not with WG's) and instead I'm after proper deepthroat which I actually prefer to sex. I'm still looking for a good one in Bristol if anyone has some recommendations by the way !!
The other point is, a regular WG who's prepared to bareback anal (perhaps not in the rare type of encounters given in theory above) or any bareback for that matter is going to be much more likely to have the nasty one !
My advice. Never do it. EVER ! Just imagine if you contracted it. If you're in any way a decent man you should feel obliged to tell any future partner or WG that you have it, even if you're planning to use a condom. Good luck in trying to get sex again !