Your husband is correct: perving is a normal guy thing (and a normal girl thing) and other guys just aren't so honest about it. The reason they aren't so honest about it is because they know it may upset their partner. So basically, the other guys care about upsetting their partner and your guy doesn't.
Regarding the perving - next time he does it, tell him you don't want to hear it. If he still won't stop, then you're dealing with a man who won't respect your thoughts - and this is a far greater issue than just perving.
And just as an aside: While this 20year old on FB may be a little kid to you, she's probably not a little kid to anyone else. Heck, I'm mid 40's and there is a few hot 20 something guys at my work. Just because they are younger doesn't mean that we can't appreciate them
As long as your hubby didn't write something such as "I want to bone you" on her page, just let it go
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.