It's only not being suggested as a replacement if you're suggesting it to fertile people to adopt instead of having their own children as well, which no one does. No one hears "we're trying for a baby" and says hey! Why don't you adopt instead! The fact it is only suggested to infertile couples literally makes it sound like you're suggesting a replacement.
No one needs to keep adoption a dirty secret, I haven't suggested that. It just needs to stop being suggested to infertile couples as a solution to their problems, because I promise you, they are aware it is an option already. Some infertile couples will choose adoption but every time it is suggested to someone going through infertility you are both insulting their intelligence and invalidating their pain. I really don't know what kind of answer you're expecting? Yes we thought about it but it didn't work out, yes we thought about it but it's not for us, yes we thought about it and we've done it. The only answer that would make it a useful suggestion is "oh thanks, we haven't thought about that/didn't know it was an option" and that is just not an answer you're ever going to hear because we all know adoption exists.
So I really don't understand why adoption is always trotted out as a suggestion. It's good that it exists, it's good that it happens, it's fine to talk about it in a positive way, it's just rude to suggest it to people who are having problems. Like lots of other things in life you would never suggest to someone because it's a personal choice.
If you want more respectful discourse about a topic, don't say hey I know this might come across as insensitive and then suggest it anyway. That's not how you get nice polite responses from people going through something painful and upsetting.