you already pointed out how they are blending in with the back drop, mostly because of her black shirt and dark jeans.

What I see:
You cropped her big toe off.
His hand position is awkward (the one resting on his knee).
I wish you could see more of his hand resting on her belly.
Is foot pointing to the camera, makes his foot look unproportional to his body.
Maybe if you would have angled them a little??

Thanks MWAC, I will always remember the 'foot' from now on. Are we seeing something different in His arm placement - I think laying on his knee like that is a quite natural position. I agree about his other hand though.

What I like:
the interaction between them, you can sense the love and joy between them.
Lighting is good.
Exposure is great.
You PP is nice as well.

You need a very large light source, close to her, to avoid those dark and sharply defined shadows, wrr.
The sun surely is a large light source, no doubt. But when you move it 93M miles away, it actually becomes "small". On a bright, sunny day, a white reflector held on the side opposite the sun, will throw light into those shadows and soften the overall look of the photo.
This reflector could be a $100 collapsible "store bought" devise, or a large piece of cardboard cut from a box, painted white. You will actually be able to see effect when you're out there shooting.
Sometimes people use a flash off-camera to achieve the same basic effect.
All this being said, your wife may not like the photo as much as she likes the one you posted. So...there you go!