I think this whole discussion is quite interesting, given that the average MN narrative is if a woman wants (some or more) children but her partner doesn't, she's absolutely entitled to leave him to find that happiness with additional children.
So what's ethically different here?
The OP doesn't give the full context, but we can suppose that to go through all that, the woman is very very keen to have children.
She's considering her future life. She knows, absolutely, that she wants children in it. So she's taking steps to ensure that happens.
The missing context is, OP doesn't say whether the testing is to promote an informed discussion, so that if there are potential problems identified, the couple can discuss them and agree approaches in advance. That sounds very sensible, although of course they could still suffer unexplained infertility etc, they're taking the most informed path they can.
Or, it might be that part of the future she envisages for herself includes a family where all the children are biologically related to both parents, which might be impossible if the man has low fertility. You might disapprove, but it's a choice she can make.
It's also unknown what the male partner's views on the whole thing are, and how keen he is for children, or bothered he'd be if they weren't biologically his, or indeed, if he would still want to have any if they wouldn't be biologically his, and/or whether he knows he could still be as committed to and loving to them if they weren't related. Given what we see in terms of inadequate fathers who are related, that's also not a small concern.
Also, since infertility can be a massive strain on a relationship, it's not unreasonable to want to be forewarned, to know that this is an issue you'll need to face together.
But, supposing she did want to leave him if he were infertile....
No, it's not his fault. And yes, he might also be devastated by it.
But there are any number of threads where the repeated advice is "You can leave a relationship at any time for any reason."
The ability to have children is a pretty fundamental reason for a lot of women. And from the importance she's giving it, she clearly wouldn't be happy if she couldn't.
So ultimately, for this person, I feel that she's actually making the ethical choice, by ensuring that this doesn't become a major issue for her down the line, and causing her to want to break up an established marriage, which would be much more painful and disruptive to all involved (possibly including such things as wider families, as well as the couple themselves) than it would be to end the relationship before that commitment was made in the first place.