This is my idea: I suggest a theme, and the next poster makes a double entendre based on that theme, then optionally suggests a new theme.
For example a cars theme: Let me drive your stick shift.
Ok, that’s pretty lame. I’m counting on my fellow dopers to do better. 
Theme: cooking
“Hey, how about a blow job in the kitchen?”
What? Not subtle enough?
Okay, okay, I’ll play nice.
“I’d love to baste your butterball.”
New theme - Straight Dope Message Board.
Um, hey baby, how 'bout I slice of a hunk of your flesh, de-bone it, let it marinade overnight in lime juice and garlic, then cook you over a hot charcoal fire, slice you up into strips, and serve you on fresh corn tortillas with Chihuahua cheese and rajas poblano and some cilantro?
Um, I’m not very good at this.
Wanna see a REALLY long post?
Next theme: spelunking (had to go with something easy to get it started)
Can I see your “post”?
New theme - Fly fishing
spelunking
“Looks good from down here, want me to climb up there?”
or
“Like the look of that crack, wouldn’t mind squeezing inside it myself…”
fly fishing
“Hon, i meant you go fishing for my fly… with your teeth!”
next:
computers
Spelunking - Hey baby, wanna explore my cave?
Fly Fishing - Check out that guy’s rod!
New Theme - Simul-Posting.
Baby, two posts are better than one!
Computers: “Is that a hard-drive in your lap-top or are you just glad to see me?”
Simul-Posting: “Looks like we finally came at the same time.”
New Theme: Suspension Bridges.
Baby, your sense of rhythm makes the whole place shake. Too vague?
It’s good struuter, but you forgot to post a new theme.
suspension bridges
“Think we can make this thing swing any harder?”
Shakespeare
Shakespeare:
“Come here baby, I’ll do you As You Like It”
painting
Painting: Primer, I just met her!
hoo that was funny.
gymnastics…
gymnastics: Now it’s time for the floor exercises.
bathing
I just love the way you stroke your soap.
Baseball
Bathing: Oops, I dropped the soap!
I’m straight, really I am…
Cabnet making.
I’m sorry…
Baseball: Hey baby, swing your bat, go for that homerun and slide headfirst into home base.
Um, easy one: Golf
I like the way you swing
Next: Star Wars
Baseball: “I was thrown out trying to steal second base.”
Next: Etymology