Short of the cancer side of things, I went through something similar when I was dating after my divorce. I actually don't mind whatever the euphemism is these days: "plus size", "big-boned", whatever, though for me, "proportion" is important. I met a woman on Match; cute, hip, funny, into music, into politics, fun... and she was clearly struggling with her weight, but it was sort of severely out of proportion. If you saw her from the waist up, as I did, it was "okay, like most of us, she is on the heavier side of the equation, but nothing unmanageable", if you saw her from the waist down... the same geometry that Chino referenced says that it would be impossible to use one seat on an airplane. Not a tight fit, but impossible. We went out a couple times, and sort of morphed into the friend zone (though not irreversibly so) and I felt I made my decision (then I met my current wife, and it seemed like things went the way they are supposed to go).
I ultimately looked at it this way: it's a monumental, but a very personal, decision. If you can decide not to see someone because it "doesn't feel right" or "you have no spark", but without any discernable reason - which happens all the time - why does that change just because you DO have a reason? How is this different than ruling someone out for any of 100 other reasons? I get that your dilemma is feeling "shallow", but you can't control what makes you hard any more than you can control the weather.
(Just asking, and I realize that condoms have other benefits than just pregnancy, but if she's gone through PCOS are condoms a necessity?)