Yes. Probably because many people use "partner" when it's actually just boyfriend/girlfriend, like they're trying to inject some seriousness when there really isn't any.
A girl I know says that marriage is just a piece of paper to her. She says the man she had a baby with, and bought a house with, was her husband and she calls him her "ex-husband". I 100% disagree with her calling him her ex-husband because, simply, he was not and their relationship wasn't that serious (as evidenced by how quickly each moved on).
Honestly... anyone can buy a house with someone. Anyone can have a kid with whoever they want (and sometimes don't want). Choosing to continue to stay together is great and all, but that choice means you're not tied legally, you're not at the same risk (half of assets, inheritance etc etc), you also don't have the same rights (next of kin for example).
To those that say they just "didn't feel the need" to get married... so why don't you just go do it? Just go sign that piece of paper and make your relationship legal. There's got to be a reason you don't want to do it. If it really doesn't alter your relationship at all, doesn't mean anything (when you know really, it does legally) then go and do it. I can't help but feel like your failure to take "that step" is because you realise that there is a certain "next level" to a married relationship and you just don't want to go there.
So yes. I do assume the relationship isn't as serious because I believe if they truly wanted to be together, "let no man tear asunder" and all that jazz... they'd take that legal step, that final step, to bind themselves to the person they love.
FWIW it doesn't mean all marriages are great (or that all non-marriages aren't serious), but most people assume that if you went to the effort of getting married, you'll probably try harder to stay together because you've got more to lose, hence the difference in answers if you're "just" partners.