Clearly from this thread, this is an experience that most people go through. Even without children, I think the first years of marriage can be hard.
I've certainly felt like this. I don't think it even needs to ba as concrete as "Ever since DS/DD came along, our marriage has been going downhill" - I just think that becoming a parent both brings new areas of your relationship with each other to the fore, and changes how you previously saw your partner (and sometimes yourself! ). We all have very deeply rooted ideas about what sort of parents we want to be, based on our own childhoods, and often we don't fully realise what our expectations of ourselves and our partners as parents are until we actually have children together.
In my case, I often feel very resentful of and frustrated with DH due to the things he does and doesn't do with DS. But he is a good father and I'm starting to realise that MY way of parenting isn't necessarily always the RIGHT way. If it's any consolation, our marriage went through a very tough time when DS was between the ages of 1 and 2, but now he's almost 3 and we have DD as well, it actually seems to be coming together more. DH is certainly a lot more help now we have 2 children.
To paraphrase someone famous, "Life is what happens to us while we're planning on something better". It's so easy to get caught in a downward spiral of resentment and bitterness over the minutiae of daily life once you are a parent. You're both tired, maybe money's tight, and it's all too easy to pick on each other. But we only have this one shot at it, so it's worth taking some time out to spend together as adults, remembering what it was about the other person that you once found so enchanting.
It will get better, but not on its own. You both have to put in the effort to be kind to one another. It may feel forced at first, but it will become habit and then you start to feel appreciated again. Don't let this drift - find a time to talk things through with your DH and see how he's feeling, too.