I think I'm lucky that my husband and I have very active imaginations which we use to role-play with each other. We'll role-play everything from sweet romantic love stories to very deviant sexual scenarios. When we get a good fantasy going, it can last for months and be just as intense as falling in love for the first time. There's no reason or need to be with somebody new because any of the excitement or benefits of being with a new person we can recreate with each other anyway.
Even if we didn't have that, I'm pretty sure I'd still choose monogamy. I'm not somebody who forms relationship with people easily. So to have a best friend as caring as my husband that I'm compatible with on so many different levels, that's something I'm really grateful for. Unless we really couldn't be happy together in the long-term, it wouldn't be worth taking the risk of losing that.
“This planet has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most of these were largely concerned with the movement of small green pieces of paper, which was odd because on the whole it wasn't the small green pieces of paper that were unhappy.” ― Douglas Adams, The Hitchhiker's Guide to the Galaxy