Your fears are justified. Who in their right mind doesn't get concerned when their husband is coming in contact with a pretty woman, old or young, with a husband or not? You'd be nuts not to, and I think its normal.
I couldn't quite read your husband's answer to your question as to whether he has a crush on her. I think you were quoting him as answering in a sarcastic way.. like "Yeah, right. She's old!"
I could be reading it wrong but I'd still plead his innocence. He's probably has a little bit of the ding-dong gene going on here, like so many husbands. I know hardly a guy who doesn't do something stupid like what he did with putting the kid between you and himself. I think it was unintentional. Or maybe he's no ding-dong at all, but very politcally savvy. If your kids are very young, perhaps he was thinking if the kids acted up in front of the boss, you or he could better handle them before an embarrassing situation arose.
I guess the big question is, besides this incident with him placing the kids between you, has he otherwise attentive to you? Or is he distant all the time? Is he not emotionally invovled at all? Is he acting really different like getting "dolled up" but not for you? Then maybe get a little worried. But don't borrow trouble. Keep your eyes open, but don't accuse unless you have a solid reason to. While I'm a firm believer in trusting your gut feelings about this sort of stuff, sometimes we can project our own fears and thoughts on to someone else or interject them into certain situations.
I think the fact he says he loves you is huge! I'd not belittle it, but take that spark and fan the fire! I think whether he's right on or not, the fact that he says you cause contention is not. He definitely is having some concerns of his own. But it dosen't mean he's ready to bolt. Maybe he just doesn't feel he can talk about it. Anyway, from what I've read, It doesn't sound serious enough for counseling. But if you feel things do seem to be heading in a not so great way, I'd consider going on a couples marriage renewal retreat. Many churches offer these programs. Or just plan a romantic getaway away from the kids so you can talk and reconnect. You've already got the man! Maybe its time for a tune up!