I admit I do have that fear with S2. He hasn't sorted out whether he's mono or poly, and I worry that after he's recovered, so to speak, from his divorce, he'll decide that he wants to be someone's one-and-only again and to have her be his one-and-only.
I try not to think too much about it. He says he doesn't consider it likely; he's enjoying having his freedom and not being anyone's sole support in any way. He likes our arrangement because he's in love with me and likes having me around, but he doesn't have to financially support me, emotionally supporting me is shared with Hubby, and S2 gets to live alone, which is something he hasn't been able to do most of his life (he grew up with five siblings) and is finding that he prefers.
If, in the future, he does decide he wants another monogamous relationship, then that's what will happen... Our relationship will end and he'll find someone else. But our relationship could end at any time for any reasons, so I try to put all of that aside and just focus on the fact that *right now*, we have a really awesome thing going.