I am the "stray single". I'm in a life place where it's far easier to meet attached people open to me as a secondary, than people who want to be a primary live-in life partner with me who are also compatible. In the former situation, there's almost an embarassment of riches - as long as they're willing to be with me as 1) a lesbian (I am not interested in the husband, or even in having him in the room) and 2) an actual relationship and not a plaything and 3) a stray single.
I am confronting my own areas where I think I would be jealous. Surprisingly, it's not over the sex or the depth with the primary committed partner.
There *would* be jealousy over not having someone to share a home with and feel like part of a couple. Which I feel would be exceedingly difficult for me to find and so what I feel is actually wistfulness and envy - watching people I care about get to have cute little homes and gardens and throw their parties as couples.
I would not be jealous of the husband - nor would I want to twist anyone away from their partner - but seeing someone I am fond of go home to that cute little home and cozy home life, and someone with whom they share finances and daily concerns, would be a bit envy-making.
Envy is a different emotion from jealousy.
I would want what she has. But not necessarily *from* her.