The thing is, you can love someone without being in love with them. People who fall "out of love" will often say this about the person they were in a relationship with -- they still love them, care about them, and hope they can find happiness. But they just can't see them as a lover, as a romantic partner, anymore. They have familial love, but not romantic love.
Honestly, as much as everyone wants to paint the dad here as a deadbeat scumbag, he seems like just a normal guy who was unhappy and realized his attraction wasn't for his partner anymore, but rather someone else.
I mean, at the very least, so far nothing has indicated that he was actually cheating. He realized he was in love with someone else, but it at least seems like he may have ended things with his current spouse properly before pursuing that any further.
It still doesn't excuse the fact that he wrecked his daughter's life in the process, but idunno that he could've predicted the sheer magnitude of the effect either..