In all honesty - I thought my "ideal" would be for myself or my husband to date someone who was already married, or who already had a 'primary'.
The reality is - that unless the person you're dating is respectful of YOUR life - and you are respectful of THEIRS - there's going to be conflicts.
For instance... if I am with someone who has a sick kid - I fully expect that the sick kid has 100% of their attention until said kid is better. And if I am taking care of my sick kid - I fully expect that my 'other' partner understands that my kid comes first.
When the secondary or other partner doesn't respect that and insists on the same amount of time and attention regardless of what curveballs life has thrown at you - there's going to be a problem.
Now - I would still be more likely to date someone who has a life... but I would be sure to observe their life to see if our lives mesh before letting it get too far.