That's not a good start, and unfortunately it's likely a sign of things to come.
You've been having sex once a month since you were pregnant, so you've had a fair warning of this, I'm sorry to say.
But the fact that you're on your honeymoon (I assume - HOPE-) without the kid(s) ) and there's been nothing... that's something to be concerned about.
The first thing that popped in my head, and it may be wrong of course, is that it's no coincidence sex has slowed down/virtually stopped since you were pregnant. 15 year age difference, so I'll guess he's ~40 and you're in your mid-20's, plus or minus a couple of years.
As we all know, pregnancies can do two things (sometimes just one) - change a woman's body, and change a man's view of her. Either of those could have put him off, rightly or wrongly. If your body has changed, that may be it. And/or he may now view you as a 'mother' first. Either way, to him you're no longer the nubile, sexual, young woman you were prior. Men can suck like that
Don't discount the fact that parenthood and all the responsibility that comes with it can affect men the same way it affects women, and some things no longer become priorities. If this is the first prolonged time you two have spent alone in a while, he may simply be relishing that fact, and sex is far down the list of priorities. Still not justified, though, but all the same.