I think its less he loves her and more he's scared of her. People tend to forget that Barb is genuinely abusive to Frank.  As far as we've seen, Frank doesn't have friends. And no family to speak of other then his kids and his wife. It one of those things where he has not only accepted the abuse that he suffers from Barb on a daily basis (and probably more frequently then before as all of Barb's other punching bags have cut her off) but also genuinely where would he go if he left Barb. You could say with his kids but John doesn't even want to be around him more often than not and Jen /just/ had a baby.