Actually I think attachment to anything is a bit dodgy. Pets and people die even if they don't leave, they are annoying and imperfect, things deteriorate or break. Disappointment and grief is unavoidable I think. I am not a huge fan of attaching to a therapist, I just don't see it as worse than attaching to any other stranger.
And in your analogy - I would think in an affair, a married person would be the best choice - they can't smother you - they have other obligations that mean they can't get too annoying.
As long as they don't want to leave their spouse or partner for you - it would seem in theory a good set up. The trouble as I see it would be if they wanted to throw everything over and be with the new person- if I was the new person I mean.