Het line with NXVIM/Scientology Cult-ey twist. My guy and his wife are members of the cult, your lady is a higher up. My guy catches her eye. Maybe she likes the way he looks. Maybe she wants him to get her pregnant. Maybe she just hates his wife. For whatever reason, she decides to manipulate her way into fucking him. He's and his wife are horrified but are True Believers. Maybe your character frames it as some kind of test to get to the next level, as a punishment of the wife for failing somehow, et cetera.
Can play with a few elements here depending what speaks to you - dub con, reluctance, older/younger pairing, public sex (maybe she makes him do it in front of the wife or other members), power dynamics, bdsm, manipulation/gaslighting, drugging/using aphrodisiacs. Open for exploration of where this might go.