Whether or not they're single or married, my basic considerations would be:
Are they supportive of the relationship I have with my wife continuing as is/growing stronger?
Do they get along with my wife Ginko well enough that we can all enjoy hanging out together, along with any other partners they might have?
Are they able to reliably see me once or twice per week without shortchanging other parts of their life?
If something comes up and they have to cancel a meetup, will they let me know as soon as they know, and make it a priority to reschedule?
Are they ready for a strong physical AND emotional connection?