A Definition

Intergroper (n) An unknown third party who intrudes on the amorous advances of others by copping a feel for himself while the others are distracted.

Sample Sentence: "Oh, Horace, you beast, you have never touched me like that before!"
"Nor am I now, my dearest Beatrice: I have discovered a wily intergroper has inveigled his way into our boudoir!"

(See also: Armed and Langorous and Schlepping with the Enemy.)