I think you have to work hard at keeping the spark, especially if there have been problems, you have young kids etc. I posted this elsewhere earlier...
"I went through a rough patch with my DP and found it difficult to get my sex drive up when I had so much emotional stuff going on, we split up after the birth of DD, I was very angry etc. You might think these are all really silly as it is a slightly different situation etc but you never know!
Things that have helped us are being more touchy feely and intimate without having sex, we have baths together and just talk, or read. We go swimming, sauna, steam room etc together. Massage and lots of it, this can very quickly descend into something erotic. Would he have a problem with you finishing yourself off as it were if this did get you (and not him) sexually aroused?
The other thing that really worked for me was very slow burning foreplay and don't get me wrong I know this isn't for everyone but texts/emails throughout the day, packing a note in with my lunch etc so by the time I got home I was really looking forward to seeing him - just because I know how much he loved/wanted me IYKWIM, not about sex at all really.
Oh and kiss, LOTS, at every opportunity in fact. This was without a doubt the best bit of advice I ever received, kissing made me feel loved and sexy."
These are all of the things we did to get our spark back and after a reasonable amount of work and effort (from both of us) it has worked. It was hard though, I have a 7.5 month old and we've been working on gettin us back together for the last three/four months, I've had a young baby, back to work FT etc so it required a big commitment.
I don't think you can expect to have that spark after the breakdown of a relationship in whatever way without proactively encouraging it.