I see your point. To a point. (!)
But I don't agree that comparing fertility treatment to cancer screening is quite right. They're different things. One is an illness that needs treatment, the other isn't. Unless we liken pregnancy to a cancer, which I certainly don't and I hope you don't either!
Second — you could make that argument about any couple, or indeed individual, who already does have access to that employee benefit. That's why it's discriminatory (or could be argued as such, anyway). I think the discussion is around what constitutes fertility and infertility, and whether surrogacy should be considered a fertility treatment or not.
It's like this: Jane and John both work for Steele's Pots and Pans Ltd. They can't have kids. Jane has a medical issue and John has shit sperm. The fact that their biological archetypes can get pregnant via PIV sex doesn't impact the fact that Jane can't, through no fault of hers or John's. At that level, John and Jane are at the same status as Adam and Pete, a gay couple who also both work at the same company. But at Steele's Pots and Pans Ltd, Jane and John get health insurance cover for fertility treatment and (it would seem) surrogacy, and Adam and Pete don't.
I don't think you can differentiate between heterosexual and gay couples in that way, based on what women in general can do (i.e. conceive). Because policies like this exist for very specific purposes; to help people who do not conceive via PIV sex.