3 kids and married here. I guess I will weigh in on this subject.
Sex was great at first, waned about the time we got married mainly due to me. Got pregnant, then we f*cked more than ever, got pregnant again, and once the second kid was about 2 or 3, we started having pretty regular sex, not quite as often, but the quality was better than I've ever had, and it sort of surprised me. It was almost like I was f*cking a different person each time, can't really explain it. Anyways, got pregnant again. I got a vasectomy after that. Since then it's been pretty down. I'm hoping it's about to kick back up because that kid is now about 3 and getting independent. In addition, I got semi-depressed or went through a mid-life crisis or something. I changed jobs and seem to be getting back on track now, but we haven't gotten back to the 2015 sex levels. Problem is, I'm 37, and I'm having trouble getting my 'desire' back up to optimum levels too. That's another story altogether (quit working out, possible low T, etc. things that can be fixed).
Make no mistake, kids running around the house, in your bed, etc. causes you to do it less. No getting around that. I mean I guess you can always go out and spend a bunch of money on vacations and dinners to have 'fun'. Biggest thing is to not get 'stuck'. You have to continue to live life and have some excitement going on, no matter how tired you are.
I don't think there's anything sinister going on here outside basic Game principles. Keep her IL high, and you'll get more sex. It will just be more on a practical type basis, because that's the way it has to be if you're trying to run a functional family.
I've found the best time is the mornings. You wake up with a hard-on anyway, and there's no pressure to perform or think or anything because you're sort of groggy, both of you have stank breath, whatever.
It's also a situation where you have to go get what you want, and don't stop until you do. I'm not saying commit rape, ok...that's not what I'm saying. I'm simply saying to be the alpha and go put the moves on her, don't wait for her to do it. This is tougher than it sounds especially in a LTR or marriage when the day to day life becomes mundane and perhaps your confidence wanes.
But there again, it all comes back to confidence, and the basic things still affect that. Nothing changes from 20 to 40. Have to be comfortable in your own skin and what you do with your time. I still say the best start to gain this back is physical exertion (exercise, working out, etc.).