I don't feel terribly excited about ANYONE getting wed, so I don't feel any 'less' excited about a couple with kids already than those who don't have kids.
That said, I kind of understand the OP feeling like this, as I have a similar thing about couples who are not married. If they break up, (when not married,) I don't feel as sorry for them as if they are married. I kind of don't take the relationship as serious. (Even if the unmarried couple had been together 3 times longer than the married one.)
Neither does society really.
Someone who works with me recently lost her husband to cancer after 7 years of marriage (10 years together,) and everyone was all over her, she got 2 weeks paid time off, bereavement counselling offered through work, the works..... Someone else lost her partner of 20 years a few months later, and no-one batted an eyelid. Also, she was not allowed time off as he wasn't a relation, he was 'just her live in partner.'