Honesty is the best possibility

You forcefully pull the see-through wrap off of Vanessa's body, and she shudders and lets out a small gasp--every aggressive move you make seems to take her by surprise, but she hasn't gone running for the hills yet.

"I love Suzy and I always will," you growl as you pull Vanessa's bikini top down, exposing her gorgeous, full breasts, so much perkier than your wife's still beautiful tits, "but what she doesn't know won't hurt her. Understand?"

Vanessa bites her lip and nods her head slightly. You pinch her nipple slightly and she gasps again, her body straightening.

"I said, do you understand?" you growl again.

She murmurs, "y-y-yes... I under--understand."

You take your hand and wrap it around the front of her neck, but you don't squeeze--yet.

"Call me sir."

She gulps. "I understand, sir."

You take your hand off her neck and slide it down her body, over her erect nipples, her toned tummy, and under her bikini bottoms, letting yourself feel her teen pussy for the first time.