LOL. I appreciate your directness. And "student-sauce" and "love-tunnel" gave me a laugh!
But in all seriousness, yes, I am very aware of how pregnancy occurs (as someone involved in sex education and with a few biology credentials under my belt)... They, however, are another story, and I've had to be very explicit about risks they underestimate. It's disappointing, because everything except her wanna be baby mamma drama is a pretty great fit for all three of us. I do trust her not to be trying to steal his semen without his knowledge, although I do not trust her in "the heat of the moment." However, I do trust him utterly and completely; I know his self-control and his commitment. Maybe I'm wrong to trust him so, but meshing your lives together for six years does take the trust thing to a very high level. He wouldn't be my long-term partner if I couldn't trust him so highly.
Where I am right now, I would rather trust them to do the right thing than run away scared from an otherwise good relationship. If they disappoint me, "cheat" after we made agreements, or she gets her hands on his sperm (in your words) through diabolical means, then I will be heartbroken, but I will move on alone and survive. I'm a pretty tough cookie, but I hope that they can find an arrangement that I can trust and respect because I don't want things to end with either of them.
Thanks for the healthy dose of reality! Your tone seemed a bit harsh, but I always try to assume best intentions, especially as it's hard to read tone from what strangers post on the internet. So I think you're just trying to give me a wake-up call, in case I needed it, so thanks!