It's called 'keeping someone on a back burner'. There are lots of people who do it.

I have an ex bf who we have discussed our possibly finally being together when we are 80. He's married, I'm married. He and I never really materialized and left seemingly 'unfinished business'. But, it's ridiculous and nothing will ever happen.

One of my gf's also has a guy who is friends with her and her h, who she says he is her back-up plan should her marriage end, and her h doesn't even care.

Now that we're over 50, meh.