我发现我的女朋友在和我的朋友发生性关系时欺骗我

A sex act is sexual contact between two or more persons by penetration of the penis into the vagina or anus; contact between the mouth and genitalia or by contact between the genitalia of one person and the genitalia or anus of another person; contact between the finger or hand of one person and the genitalia or anus