I wouldn't go quite so far as to say that all opinions are valid. I mean, heck there are people who think Nazis are good and child molestation is ok. And those people need to shut their pie holes if you ask me. But with normal non-Nazi people, yes, I think people's opinions are valid when it comes things like people who have never been in a relationship giving their opinion on relationships. Because, even if you haven't been there yourself, you're still aware of how relationships work, what's healthy, what's not healthy, etc, and you can still use your imagination to help you decide how you would react if you were in a certain situation. So I think you can give your opinion about anything; I don't think you're limited to only things that you have personally experienced.

I think no opinion is invalid, as it is an opinion, not facts.
But I also think that experience makes a difference. If you are asexual and never had sex before, then you have a certain view about sexual things that may or may not be real.
Someone who never saw a cloud can have an image of what it is from other people's description, or look up on the internet what it is. That image can be spot on and they can be an expert, or it can be totally wrong but they don't know because they've never seen it.

Everybody's opinion has a value imo, but not always the same value.
Depends on the knowledge and experience people have + their attitude. I'd trust my mother on stuff about educating children. I would trust an expert with no kids about it if they're being open an honest. I wouldn't give a lot of value to someone with no kids that is being judgemental about it (= you do this wrong, you should do this).