That is exactly what happened to me in the 80's when I got married.. Since that time 27 years ago to be exact, 2 kids, big dog, 10 hour plus a day job, and all the responsible trimmings that go with it, I can honestly say I relate bro..
I've gained 60 lbs since that fateful day back then, (I was 19 however) and not all of it is excess, but I's say a good 12 plus lbs could be stripped in time, but my point is that you'll get your motivation back. I have gone through many motivation 'cycles' (non a.a.s) with this final one being the best since my late teens. Sort of a positive mid-life crisis for lack of a better term.
I hope you get your motivation back before 27 years, but I have improved mine even more by setting very realistic goals for myself. I realized that (going way back in time now) once I'd married, and my 'dreams' of being like the guys in the magazines were dashed, I think I just drifted with no goals, etc.. Always worked out, but no fire burning. This year I turned 46 and realized that I wanted to feel as close to the excitement I felt in my teens as I could, so I turned my old dreams into more realistic goals.
It's hard to explain, and may only be my gig, but I just am working to be the best that I can be with what I have, and not trying to be someone I could never be. I begrudgingly (long story) bought a tiny Ipod music player and downloaded the different types of music that aided my moods for how I want to lift in any given day, and also picked some music from my teens that I recall having great workouts listening to at the time. The first few weeks were hell, and I almost backed off, but then I started noticing some changes that felt similar to how I felt back in my teens. It's not the same, and I'm not re-living the past, just adding some old tools to go forward.
I am not making this an 'all about me' but rather letting you know you have to find that feeling, motivation, reason, whatever, and make it very important and a priority to you. If you can do that, and stay committed, you'll be back in no time.. After 27 years of marriage I can honestly say I've felt a lot of "lack of motivation" at times. It happens to most I should think.
Good luck, and just do it bro!