Characters
If I wasn’t happily married and you weren’t my wife’s stepsister I’d have an affair with you in a second!
— Sidney • The Object of My Affection

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker flirtatiously admits an attraction, saying only two obstacles stand in the way: their happy marriage and the family tie to the listener. Without them, they claim they would begin an affair instantly. It is compliment and confession wrapped in a conditional.
Context reviewed