and to think there would probably be NO or very few extra marital affairs if it didn't?
been with DH 6 years and married 3. we have 2 dcs, 7 and 4 (my dc1 is from a previous relationship) i love and still fancy him, he is a brilliant husband and dad and he says he loves and fancies me as well. but it is just not the same as the early days
we kept our "spark" for ages, probably for the first 2 or 3 years. which is certainly longer than it lasted in any other relationship i have ever had. we have both had boring sexless relationships before we met and for the first few years could not believe our luck in finding eachother and how good the sex was, at first, we were absolutely crazy about eachother, it was almost like an obsession
dh says the "spark" is still there, for him. but i don't believe him TBH. we go to bed and just want to sleep. inb the last few months, we only ever DTD sunday mornings when the dcs are downstairs watching tv. with one eye on the bedroom door. once a week ffs. a few years ago it was every day. and we would feel sorry for those "boring" once a week couples and now we are one of them :( and i am gutted tbh as we went away last weekend with no dcs and did not even do it once. 3 years ago if we had gone away just the 2 of us we would not have got out of bed 
it doesnt help that i am 5 months pg and feeling pretty unsexy anyway. but when i was pg 5 years ago with our first DC together we still couldnt stop shagging right up till the due date (sorry for tmi
and resumed it quickly after the birth
anyway i wish nature allowed us to keep feeling the way we do in the beginning, because i think then everyone in relationships would be a whole load happier :/