She lightens my sadness,
She livens my days,
She bursts with a kind of madness
My well-ordered ways.
My happiest mistake, the ache of my life:
You must meet my wife.
One thousand whims to which I give in,
Since her smallest tear turns me ashen.
I never dreamed that I could live in
So completely demented, contented a fashion.
So sunlike, so winning,
So unlike a wife.
I do think that I'm beginning
To show signs of life.
Don't ask me how at my age one still can grow--
If you met my wife, you'd know.
- Stephen Sondheim, A Little Night Music
I have one major (and thus far infallible) rule for dating married guys: Always Side with the Wife. This probably seems...weird? Stupid? Counter-intuitive? Welcome to nonmonogamy.
In any set of three people, the game is eternally two against one. In the beginning, it will be New Girlfriend & Husband versus Wife (this reflects my heterobias, of course - please adjust genders and titles as appropriate). Often Husband is oozing new relationship energy, and doing his best to negotiate as much time as possible with Girlfriend without making Wife feel anxious/annoyed/hostile/threatened/put-upon/sidelined/etc. He will almost inevitably fail, regardless of good intentions. Opening a relationship is hard, even when everyone is GGG.
This is reason #1 to ASwtW: Wife's discomfort can end your new relationship before it begins. If, for example, she has misgivings about how regularly you see Husband, you'll often do well to acquiesce. Try to accommodate any [reasonable] requests, and encourage Husband to do likewise. As you and Wife become familiar, she'll see you're not a threat to her marriage. You may even reach a point where she looks upon you as part of her family.
Which brings us to reason #2 to ASwtW: eventually, the game becomes girls against boy. As the women get comfortable with one another, and as the secondary pairing loses some of its Shinee™, every dating triad I've observed and experienced drifts into Established Girlfriend & Wife versus Husband. The women commiserate over his shortcomings, and comfort one another when one relationship or the other (or both) hit bumps. I wince as I use the term, but the women often develop a sort of 'sister-wife' relationship, minus the religion and unflattering couteur.
Personally, every time I've dated a married man, I've ultimately bonded more with wife than husband, and my friendship with her often outlives my relationship with him. Knowing the likelihood of this evolution, I go out of my way to connect with any partner's wife.
The spouse is your ally, not your competition. She keeps him balanced and centered, you offer novelty and often excitement. A healthy connection between you and she can only enhance what you each have with him.