There are a couple of ways to work through this. The first thing to do is to word your discussion in such a way that adoption comes off the table. Something like, "We were planning on having children, but apparently are unable to. Since we have no interest in adoption, it looks like we aren't going to be able to be parents." Yes, you bring it up, but that shuts down the (well-meaning) discussion of an option that you have already considered and do not want.
Otherwise, if someone else brings it up, I'd respond with, "Thanks for mentioning it. I've heard that option from a lot of friends and it's not something we're going to pursue."
After either of these, change the subject immediately. Talk about weather. President Trump (that will make people forget adoption!). Sports. Anything that will get your friends to stop trying to help you. Keep in mind that they mean well and probably have no idea how many people have already suggested the same alternative, so being gracious, no matter how hard it is, is important with your friends.