I’m in the process of putting a book proposal together . . . which naturally gets me to thinking about the word. As a kid, I had no idea the family of “propose” words could mean anything other than asking someone to marry them–until, of course, I read or watched something where their was a comedy of errors around this very thing.
But in fact, marriage didn’t get attached to the word until the mid 1700s. Propose, however, dates from the 14th century with pretty clear etymology: pro (forth) + poser (put, place) = propose (to put forth).
 
Proposal itself didn’t get tacked on until the 1650s, but at that point it still had that more general meaning of something put forth. Like, say, a book idea sent out to publishers. 😉 
But in 1749 this idea-put-forth came to mean “an offer of marriage.” And in 1764 the verb caught up with the noun, and propose became the act of offering a proposal of marriage. (Funny that it took 15 years for that, isn’t it? LOL)
So there you have it–unlike with some words that completely surprise me with their order, in this case the more general idea came about well before the specific one that has become most popular. And now off I go to get my proposal of literary genius (ahem) finished up!