'I've read a LOT about poly for years, but the stumbling block we have right now is...we are doing fertility treatments (part of why our relationship is on the rocks - infertility can kill any relationship). The rules for clinics where we live is that we have to be in a strict mono relationship and we have to sign agreements that we are, just because they are working with our bodily fluids and for infectious disease control reasons, etc. I totally understand and respect this, since it's a delicate situation dealing with embryos and such, especially since the lab works with other couples...materials, we don't want to jeopardize the safety of anything for anyone. I mean, we would be 100% safe but still...can't risk it considering we signed agreements.'
I wish you success in your fertility treatments. I have a dear friend going through something similar. It is so hard.
But I have to comment that your clinic's policy of making you sign a mono only relationship agreement to 'protect' their staff, that strikes me as bullshit. Basically they are saying that only mono, possibly just straight, couples get a shot at babies. Do they refuse to assist single women or men? LBGT folks? Is legal marriage also required?
I agree that disclosure and testing for STIs of their patients is a wise precaution both for their patients and staff. STIs can affect fertility and some can be transmitted by blood or other fluids. Increasing number of partners can increase risk but that is true regardless of relationship style. A monogamous person serially dating can rack up some serious numbers. Unless there is some rational reason I have overlooked, this is particularly insidious slut shaming. I bet they will serve cheating couples who lie but an honest polyamorous or other ethical non-monogamous folks are right out.
Please understand I do not condemn in any way your decision to sign and start treatments. One must do what one must do sometimes. You have to do what is right for you. But wow, this makes me angry.
And to attempt to answer your actual question, you could frame it as 'I'm not ready to give up on fertility treatments yet, but I've met someone who makes me wonder what if we decided to open up our relationship? It's not serious, I have not made any moves - we are just friends. But I wonder and wanted to start talking about it with you.'