OP, you might like to look on relationships for a thread about still not being up for sex after 9 months. It was active a few days ago. Lots of good advice there.
It is very very easy to devote all your emotional and physical energy into your children, to the detriment of your dh/dp. It is also a fact that the two most common times for a marriage to end are before the children are 3yrs old and when they leave home/you/he retire from work.
A baby rarely 'brings a couple closer' it's bloody hard work. A traditional relationship with the mother at home for the first year focussing on baby, involved in exclusive activities like breast feeding - will without a huge amount of effort from both parties, result in the dh/dp feeling excluded to varying degrees. The mothers unconditional love can feel as though it has 'transferred' to the baby at the partners expense.
A partner will try and push for reassurance that the love is still there. To most adults, the manifestation of love within a relationship is to have sex with one another. No sex, constant rejection/ avoidance (I was a class A avoider.. I would have headaches/stomached aches,/fall asleep with the kids etc etc ) will eventually end in one of three ways. 1) He will just accept that the marriage will be sexless. (Rare) 2) He will discuss it, hopefully you will make a plan to improve matters (I am talking about not 'feeling like doing it' not advising this at all if there is a physical pain during intercourse, that needs GP intervention) and having made a plan, also made a decision to leave the relationship if matters don't improve. Or worst of all, 3) try to discuss, wife gets angry/feels pressured/defensive , give up and have an affair.
I am not justifying a man having an affair because his wife is knackered, sleep deprived, feels unsexy and uninterested in sex. In an ideal world, a man can understand why her focus has changed, shares the burden where they can and does everything in his power to support her. Sadly the real world is not always like that. Sleep deprivation coupled with sexual frustration and constant rejection are not the ideal ingredients for 'reasonable behaviour'. The best advice still seems to be 'fake it till you make it' (if you want to get it back on track) . Or split, if you don't.