Here’s what I’ve read, for what it’s worth.
Back in the good old days, they didn’t go much for dating. A caveman would just grab a woman he liked from a nearby cave, and head for the hills. Proposal? Ring? Ceremony? Nah, didn’t need it.
Supposedly, the “best man” was his buddy who helped him get the girl. The honeymoon was the period of time that he was laying low, waiting for the girl’s family to give up on getting her back.
I’ve actually seen this in several sources, though it sounds unlikely to me. Most likely more entertaining than any other explanation, in any event.