But numpty who are these people who just jog along dully forever? I'm not sure they're as common as you suggest. I still remember the joy and surprise of discovering I'd fallen wildly back in love with DH after a fair few years of living like over cooked veg.
The advantages were: no kids were hurt, no hearts broken, no homes broken, no messy, painful divorces or court cases or negotiations on childcare, no guilt, no bitter recriminations, no sense of failure. Falling in love with him again was a real excitement and surprise, and he was thrilled by all the attention and the good moods.
The point I'm making is that people who jog along dully and do nothing to revitalise their marriage are being lazy. If they have affairs, that's lazy too. It's not the marriage that's necessarily at fault, it's the attitude. If you fill your life, willingly, purposefully, with small and large things, daily habits and bigger events that bring you close together as a couple and as a family, you'll be too busy enjoying what you have to scuttle off and look elsewhere. It takes effort, but that effort yields far greater rewards than the quick rewards of an affair.
Also, I don't know how to explain this in a way that conveys it fully, but the strength you gain from having survived so much together creates a love that is really strong, like the deep roots of a healthy tree. A lot of what people say is not worth having in a marriage is the dull, humdrum stuff. But that stuff happens anyway in life, married or not - money worries, family problems, health, work, friendship issues, concerns about the kids - they can preoccupy us singly or as couples. I know I am so thankful to have DH there to discuss this stuff with and joke about it when it gets too much.
Those positive things I listed that you say you can provide single handed, well yes, and you can also survive the tribulations single handed too. But it is so lovely to have someone constant at your side to ride them out with you too. Someone who is on your side to come home to, someone who takes over with the kids and housework when you are knackered or ill. Someone who will listen and advise when you are struggling with a key decision or telling you to stop twining on when you are obsessing unnecessarily over a small issue. Someone who says - now you need a break - have a drink or go for a walk. Someone who is there to put the DC to bed while you go out with friends, or there to stroll up to the cinema with when you have a sitter. Ordinary life. Nothing fiery about it, but it is healthy affectionate every day.
I've been so wildly in love before it made me almost ill. I couldn't function in real life because being with this lover was so intense. No way could I have made a happy long term marriage with that man, adorable as he is - and we are still friends. There is a gorgeousness about quiet, reliable love. It doesn't have to mean dull. Nothing beats it.
I think small things add up to real, worthwhile, valuable love, immeasurably better than going it alone and having an occasion hot fling that you break up when it starts to get humdrum - but maybe you don't. No idea if the OP is still reading but I don't think we've gone off topic at all. If the OP feels some love for her husband and children she needs to refuel it with the kind of attention she's been giving her lover. She'll be amazed how much more fantastic a marriage is if you put a bit of wholehearted effort into it.