I can only speak for me and my own social circle, but men who are taken and who are responsible fathers and loving husbands are always somehow more attractive than the single guy I meet out and about. It’s something about him being unattainable or with another woman that makes people go, “Hmmm, OK, he must be something to brag about.” That doesn’t mean women will jump on him to try to best the woman he’s with (you couldn’t pay me to do it — hard pass, I don’t want anybody else’s anything) but I’m not surprised that women swivel their heads at him.
If their marriage is healthy and he’s happy, my guess is he’s also wearing that confidence on him, in addition to good looks. It happens. It’s why women sigh and cringe when that hot guy is wearing a wedding ring or a woman sits next to him when he was originally sitting alone. I would’ve been far more surprised (from your description) if you told me he was single.