Everyone is making some very good points here, on this all-too-often discussed topic, even if somewhat abrasive. So, I'll throw in my two cents.
I can't speak for the mistresses, but what I've observed is that they actually aren't turned off by sex in general. I think the number one reason most BDSM providers in the Bay Area don't do extras is because of LE. Busts do happen. Why risk it?
I would say that another reason, probably equally important, is that they don't want to do sex for money for personal reasons. I can't speak for women since I'm not one of them, but I've known enough women to say that sex is an extremely personal thing for them. It's not like us guys who just want to stick our wanger into any wet and willing hole. I've had many women tell me things like "If you don't love your man, you can't spread your legs for him." Women also want to "save something special" for the man/women they love. If they're givin up full service to any Tom, Dick or Harry who can afford a few hundred dollars, it would tremendously devaluate the experience with the person they're in a relationship with.
I would say that yet another reason most BDSM providers in the Bay Area don't do extras is for health reasons. I don't think it's possible to really do 100% safe sex. Condoms aren't foolproof and there are other ways to get an STD that don't don't involve condoms. Many providers are in relationships and don't want to "bring something home" from work and give it to their partner.
Non-extras BDSM providers are a bit like strippers - what they do is sexual, but not sex. Thus, the ones that have partners are more likely to be able to do it without making their partner insanely jealous, although this doesn't always work for strippers.
And I won't even get into the MORAL issues involving sex for money, although I would tend to agree that even non-extras BDSM providers are in bit of gray area when they do things like strap-on play, cock-and-ball torture, ass worship, etc. However, I don't think the BDSM providers are semen squemish; I've had someone from Fanatasy Makers tell me that they wish hand jobs weren't illegal...
That said, a PL can't still get what he needs if he knows where to look. It would be a total waste of time to try to convince a no-sex BDSM provider to do extras. You're just gonna piss her off by asking. So, here's a few options:
0) Find a BDSM provider who allows extras. Admittedly this is a bit hard since they don't want to advertise anything illegal, but this board should help somewhat.
1) Find a full-service escort who is willing to learn the ropes (pun intended) of BDSM. This can be especially fun since she'll be fresh about the whole thing.
2) Go to a brothel in Nevada. They do have both sex and various BDSM activities in their menu.
3) Go to a brothel in Europe. Culturally, I think prostitution is less of a taboo there than here as there are more places where it is legal, or pretty close to legal. So, more women can do prostittution there with there being less stigma attached to it, and thus the women are less likely to lose self-esteem over sex for money.
4) Get a girlfriend (what a concept), and teach her the ropes.
5) Go to swingers clubs and BDSM clubs like the Power Exchange. The main difficulty here is that you have to be invited to play, and if no one wants to play with you, you're shit out of luck. But you never know what scenes you can get involved in just by being an innocent bystander.
6) Hire a full-service escort to go with you to a non-extras BDSM provider. If you're there as a couple, a lot of BDSM providers will allow sex between yourself and your partner. You don't even have to tell the provider that you're paying for your partner. The BDSM provider can do things like spank you while you're banging your ho.
Hope this helps.