M.,
I just celebrated my 15-year wedding anniversary, yesterday. I agree - there are always ups and downs, some big, some small.
It's funny, because the day or two before, I caught a guest on a radio show, who wrote the book, "All Men See All Women Naked," or something like that. Later in the interview, he gave advice on how to "Divorce-proof your marriage." The DJ chick laughed, and said, "have sex all the time, always look sexy for your man, and cook him great dinners every night."
He laughed and explained, while that would be nice, his best advice is: 1) always be polite to one another, 2) do not demean or berate each other (calling each other names, or constantly nagging about stuff), and 3) though sexual touch is definitely a good thing, don't forget the non-sexual touches, like holding hands, rubbing of the back, etc. That physical touch is an easy way to stay connected.
I couldn't help but chuckle. . . my hubby's been really stressed out, lately - working O/T on a big project, and one of our favorite things to do lately: I scratch his head. :-D
It's funny. . . we'll be talking about something, and as soon as I dig my nails in his hair - he's at a loss for words, and has told me how wonderful it feels (once he can talk again).
My point in all this: I understand first-hand how stressful it is to keep the kids alive, not bleeding, and entertained all day.
For some reason, the first year with our second daughter was much more stressful to us than the first time around. Either way - having a new baby at home ALWAYS puts tension on a relationship, no matter how strong it is. Be sure to make "Mommy/Daddy Time." If you can't do date night or an overnighter (we never felt that we could, because of the whole breastfeeding thing), when you put the kids to bed, curl up on the couch with him, and ask him about his day (or scratch his head for him). :-)
Understand that as sole breadwinner, he's stressed, too. Most of your attention is on the kids, so it's very likely that he's feeling neglected, too, even if he can't verbalize it. I have a feeling that if you take the first step and can give him that little extra attention (and remember, it doesn't HAVE to be sex), it'll help both of you feel better.
Best of luck!
M.