So my boyfriend (bi) and I (female, not sure if I'm straight or bi) are in a monogamish relationship. We were hoping to try a threesome, and in particular a MMF threesome. But we have some practical concerns, mostly about possible pregnancy. We are not presently in a financial situation where kids would make sense, but we hope to be in such a situation fairly soon—and I've got a limited time window to work with, as I'm already in my forties. So we don't want to do anything that might even somewhat reduce the odds of me popping a sprog as soon as the situation changes. And to add a further wrinkle, I have a latex allergy. I also have reason to believe that I might be more than typically fertile, even at my age, and consequently I don't want to just trust to luck.

So are there other birth control measures besides (non-latex) condoms and our "third" not actually coming while inside me that will not interfere with my long-term fertility in any way? What are the actual odds of an unexpected kid from a single sexual encounter if the condom breaks? Any other thoughts or advice?

No Babies Quite Yet

If you use non-latex condoms and the "pretty effective" pull out method during your MMF threesome, NBQY, your chances of getting pregnant are extremely low—and that's even before factoring in your age.

It's a fact that non-latex condoms have less give—they're less stretchy—than latex condoms and are therefore a little likelier to break during intercourse. So why not add some duct tape to the belt (non-latex condoms) and suspenders (asking your third to pull out) you're planning to wear, NBQY, by picking up some over-the-counter emergency contraception? You can purchase Plan B at pretty much any pharmacy and then, if the condom breaks (or even if it doesn't), you can toss those pills back and breathe a little easier.

Finally, NBQY, if you're looking for a set-and-forget birth control option, consider getting an IUD:

One of the awesome things about IUDs is that they last for years, but they’re not permanent. If you decide that you want to get pregnant, or if you just don’t want to have your IUD anymore, your nurse or doctor can take it out. After your get your IUD removed, your fertility goes back to normal.

I wouldn't recommend going latex- or polyurethane-free with thirds, randos, very special guest stars, etc., just because you have an IUD—if you should get one—since IUDs don't provide any protection against sexually transmitted infections. But an IUD would make it possible for you to go bare with your primary partner (after he's tested, if he's safe with others) without risking pregnancy and without doing any harm to your fertility over the, er, longterm.