› firstname.lastname@example.org 
Dear Andrea: My boyfriend and I have talked about doing a threesome with another woman I'm actually the one who really wants to, but he does too. Since we can't think of anyone we know who would work, we are thinking of placing a classified ad online. I've never done anything like this before, and was just wondering if you have any advice, like how to make it go smoothly and not be weird. Also, do you really think dental dams are necessary to make sure we don't get diseases from her? I am excited but also a bit nervous.
How ... refreshing? The threesome idea usually seems to originate with the guy and have a whole lot to do with his "two chicks" fantasies and very little to do with the chicks in question, so they end up putting on a half-hearted show based on porn scenes they've watched, often also half-heartedly. Way to have some half-hearted sex, and often a big fight afterwards, especially if the guy manages to enjoy himself too much despite all the half-heartedness. Of course there'll be an even bigger fight if you enjoy yourself too much and he doesn't, which has been known to happen, so you might want to talk this through together a whole bunch before you do anything.
We would now be moving on to the safer sex part, but I'm a bit distracted by my lack of faith in your or anyone's chances of finding an appealing, willing girl online you won't have to pay. It's a seller's market out there,and hot girls who want to have a threesome are rarely reduced to combing Craig's List for takers. All they really have to do is get into the habit of making goofy jokes about threesomes every time they hang out with their more attractive partnered friends, especially when there's drinking involved. Things happen. In fact, most group sex that actually happens just happens. The "exhaustive plans were made" kind does exist, of course, but more often there's some drinking and goofing around and some dancing and maybe a game of Truth or Dare or something stupid like that, and ... things happen.
So. Are you absolutely sure you don't know someone? Group sex is not only more likely to happen among friends than with strangers secured for the purpose, it's also more likely to be both safe and let's not forget this part fun. If there's no chance, like because all your friends went to church camp with you and you're positive you're the only ones who've acquired new interests since then, how about making new friends? Join an erotic writing circle or go to readings or take some classes at the local nice dildo store. Go to the edgiest nightclub in your area for Fetish Night. Most of the people you are likely meet at these things will either be deadly dull or extremely yucky, but not all! I used to go to stuff like that, and I met some nutty folks but made some ... friends too. Remember the old song: "Make new friends, but keep the o-o-ld. One is silver, and the others will have sex with you."
Now let's say that works (or doesn't, but against all odds you find an appealing prospect on Craig's List), do you have to use dental dams? Absolutely not, but that's because they hardly work and are horrible. You will certainly want to use condoms (and so will she not wanting to, under these circumstances, would be a crazy-person warning sign). You could use plastic wrap for licking things, or not. Going down on girls is never ever going to be a good method for contracting or spreading HIV, but you probably don't want to either get herpes or spread any herpes you may already have, so you'll either have to not do anything that brings a lot of wet parts in contact (unlikely), use plastic wrap, or rely on a pre-interview, trust, intuition, and Purell in whatever combination feels right to you. I wish I could tell you exactly what your risks will be, but barring the acquisition of a long-distance, anonymity-breaching virus-detection gun (and what would I pay for one of those), I just can't.
As for advice on how to make it go smoothly and not be weird, well, it IS weird. But choose someone sympatico, someone with whom you can discuss both what might happen and what just did happen. Give everyone the explicit power to halt proceedings for any reason at any point. Have a drink but not six, and agree ahead of time no hard feelings all the way around if it doesn't go perfectly. Expect it not to go perfectly. This experience may bear a superficial resemblance to porn, but porn is so ... porny. You should expect real life to be bumpier, less predictable and, one hopes, more fun.
Andrea is teaching Sex After Parenthood at Day One Center (www.dayonecenter.com ), Recess (email@example.com ), and privately. Contact her at firstname.lastname@example.org  for more info.