Wow, Cedar, you seem to have an extremely blinkered view about others’ relationships, presumably based on some poor experiences of your own.
I think there are many types of relationships that can work. I’ve seen arranged marriages work, and ones that initially started out of convenience and grew into love. Many many people DO actually find someone that they have the stereotypical mutually passionate and lustful experiences in the beginning, Cedar, (me included) but if they’ve chosen each other well this then develops into a very stable,respectful and more comfortable long term relationship for many many years after till they grow old, with shared values and interests and similar senses of humour etc. Best friends, essentially. And there are many ups and downs along the way which if you genuinely love each other, will want to work through, even if there are whole YEARS which are not great between you. But usually each partner is ultimately still the other’s Favourite person, and so no, you don’t develop crushes or want an affair or find the other dull, not permanently anyway. My DH of 22 years is still the person I would most like to spend time with, over anyone. I have never found him dull and would be horrified if he found me dull. although I think I would probably realise if he felt like that, I would think it’s extremely hard to hide long term. people do tend to notice a bit of an eye roll, or a huff or a tut, or a “hmmm? Sorry, what?” said in a disinterested tone.
It sounds like OP skipped straight to the second, settled part without experiencing the “shivers up the spine and fanny gallops from his hand brushing your knee accidentally”, which is sad because that’s an exciting stage, but even so I’ve still seen relationships that don’t start with that work out long term because there are no crushes and they are best friends with the shared qualities I just mentioned. Likewise there are relationships that start out like that and people confuse the lust with love but if they thought about it their choice of partner is wholly unacceptable. But they’re blinkered and fancy a wedding so off they go but by the time the day comes round the cracks are starting to show but they feel like they have to go ahead now anyway. They are actually “settling” too. There are many ways to “settle”.
I think, though, ultimately, the fact that OP finds her husband a bit dull is very telling. once the children grow up to mid-teen stage and need you less but are around more, and middle age sets in, with the pressures of older parents and maybe stress at work, relationship problems become more apparent. Finding your partner dull in those circumstances and having a crush on someone else sets you up for an affair. You may find that you yearn to find your “true” partner, if you feel you’ve settled for years and feel flattered by any attention that another man shows you. What if you tell yourself that the crush might be “the one.”