You're mostly right. Young couples don't often do this. But it does happen. I know three such couples. They're all between 19 and 25 years old. And two of them are officially married.
Well I assume they might have the specific kinks that would lead to that, like kink of wanting to see your gf / wife fuck someone else, or perhaps they have been in relationship for several years already even if young, or they might have be bisexual and thus most flexible in term of partners, or just the exception that confirms the rule. I know for myself I wouldn't mind if I had a gf that wanted to have sex with other women, but I'd mind if they wanted to have sex with men, I suspect its a question of personal attraction, I wouldn't mind my lover having sex with someone I find attractive myself, so I assume a couple of bisexuals might actually have a far easier time to contend with that sort of activity.
But ultimately what I wanted to point is that 'adding spice to your relationship' isn't something that should happen for young couples, maybe some other motivation might justify it, but a fresh relationship shouldn't be lacking that much already for them to seek out new venues of excitement. Or perhaps young adults these days are that much jaded by reality that by the age of 20-25 they are already bored with the normal stuff, I mean nowadays with the internet, PornHub, and sites like this, its likely kids are getting a sort of accelerated sexual experience that make them reach the equivalent to a mature couple way earlier.