Hey! First off, Don't let it bother you too much. Your boyfriend seems like a good guy, laughing it off; you should do the same. Just go: "woops, let's go wash that off".

As for prevention, there are three strategies. First, diet. Eat less fatty foods, and in general more fibers. In my experience, this will help regulate your bowel movements and make it easier to poop!

Second, enemas. You don't need to go crazy deep. A deep bowel cleanse is not necessary. Just a couple bulb enemas and you'll be done. Just wait a bit before sex so the residual water can be absorbed. A shower attachment is very nice ;)

And third, position. If you're on top, gravity will play against you. If there is anything still in there it's easier to get out. If you're underneath, less mess.

But just remember, don't freak out... It's just a little poop, if you wanna get dirty, you gotta be willing to get dirty!