Read other letters about this article
If he really loved her and really wanted to be with her then he'd let this go, or at least make the effort to (that is, he'd do couples counseling). Instead, he just wants to wallow in his self-made "rule" of what is right or wrong, and make the relationship miserable for both of them. That's a red flag.
She has *nothing* to be ashamed of here anyway. She'll be much happier with someone who agrees.