They assess love relations poorly and end up in these gray zones. They try to use reason to solve relationship issues and it makes no interpersonal sense--can even be farcical.

My approach when I met ISTj and he was in a muddle with someone he "thought he'd found a way to marry" was this: even if I never see you again, for the lovva god don't marry her ... It will be a disaster.

And:

If you want to be with me I will not accept this unresolved wishy-washy stuff. Either you're with me, or you're not. I don't have the time nor patience for anything else.

It forced him to make a choice. It was for his own good anyhow.