I mean, that does make sense to some degree. But is it really worth it to have a teen and a tween in the house just for appearances? Those ages are so hard, the kids get so emotional and often can't deal with those emotions. Two girls of that age, not matter how much they loved each other and how well they got along with each other, had to be fighting and bickering with each other.
Unless he was leaving super early and getting home super late, he had to spend time with them. Plus, both of their fathers were very present in their lives. Not all men can or want to deal with that. They were dating for about a year, seems a bit early to move in together with two kids. Was Nichol talking about having another child? At 37, she might have been able to. If he wasn't ready, she didn't have much time, maybe he didn't want kids at all. Their fight could have been about so many things, maybe it was something petty and not super important.
Their whole relationship and how the two girls fit in their life together is just odd to me.