Honestly, I would expect DH to stick up for me.
In fact, I know he would because it has happened.
When we had been together for about 2 years and were engaged, we went on holiday with two of his friends. I thought we were all getting on fine, but one night I went to bed and DH and one friend stayed up talking.
Anyway, I couldn't sleep, and unknown to them I could hear their conversation through the window. Dh's friend starting saying how he thought I wasn't the one for DH, how I was immature and just not right for him at all.
I was just frozen with the shock of it, it was awful.
I was dreading DH saying something in agreement!
But he didn't. He said "you're totally wrong, she is wonderful, I love her and am totally happy with her". He also pointed out how I had happily taken care of their children while we were on hols, and had done everything to fit in with their holiday.
Anyway, I stormed out there (it was v dramatic) and confronted the friend and said how angry I was that he would say these things behind my back. DH totally agreed and said that we would leave the next day.
It was rather a defining moment in our relationship and made me realise that I could always rely on him, and we have felt like a team ever since then. (And I think I was the right one for him as we have now been together for another 11 years very happily!)
BUT, that doesn't mean that there is anything wrong in your relationship! Some (most?) men don't like confrontation at all.
I would maybe say something to him about it, but also appreciate that he probably feels awful at not sticking up for you. It's a hard thing, to stand up to your parents.