I have to say, it is difficult. My wife joined for training when I started teaching. We were together well before that though, and still it was difficult for both of us. While on the tatami their is no hubbie & wife, but teacher - student. You cannot give a different treatment to her, or in your case expect that. While i generally will push women less hard in pressuretraining (unless they're clearly up for it), it's important not to be too soft on them either. That would not be respectful of their capabilities. That said: if the woman you love is groaning in pain because of a bad fall or a punch that connected: it's nearly impossible not to be affected by it. The trick is not to act on that emotion any differently than you would if it were somebody else with the same characteristics (by which i mean that obviously if you know a student has a bad back for example, you won't expect him or her to do backflips).
On her part, the difficulty was that while in our relationship we discuss matters in an open dialogue, on the tatami there is no room for discussion. If the teacher corrects your technique, attitude (eg. no leaning against the walls, etc), or etiquette you accept it and say "hai sensei!". The complete opposite of what we do at home (at home I get to say "yes honey!" and accept it

.)
However, the strict discipline on the tatami does help to maintain a healthy way of keeping both worlds seperated. It's clearly a different paradigm of dealing with each other, just because of these rules and the strict discipline. It makes things manageable, and disconnects both. I think if training would be very informal, and on first name basis, you'd easily revert to calling each other lurvnames on the mat too, and thus, if discipline or correction was somewhat enforced; it would be much more personal too, and you'd probably end up taking that feeling home after training.
Anyway, I've been teaching her for about a year now, and things have started to fall in place, finally. The main difficulty was my strong inclination to prevent her from getting hurt, and thus doing her short by the false assumption she can't manage herself, which she can.
However, It's great to be able to teach something you strongly believe in to your loved ones, but it's also exceedingly demanding and taxing. (not to mention tricky).
My advise to your situation though: be very, very careful.