I would like to better understand what exact physical acts constitute adultery between a married woman and a man that is not her husband?
It is adultery practically speaking: she is forbidden to be with her husband (and can't marry her adulterer)
- If a married woman willingly (not raped), (if her husband is a Cohen even not willingly) let
- A man (even not Jewish*) not her husband that is older then 9 years
- the corona of his erect penis enter her vagina or anus (for karret and/or death penalty)
*igres mosher even ezer 4.44 explains that even if her husband askes her to do it with a non Jew still you should forbid her from her husband
A person who inserts the corona into [the woman's vaginal channel] is referred to as one who "uncovers" as [Leviticus 20:18] states: "He uncovered her source."28 A person who inserts the entire organ is referred to as one who completes [intercourse]. With regard to all the forbidden relations [mentioned] by the Torah, one who "uncovers" and one who "completes [intercourse] are [equally] liable for execution by the court,kerait, lashes, or stripes for rebellious conduct. Even though the man did not ejaculate and even if he withdrew and did not complete relations, [the man and the woman] both become liable.29Whether a person engages in vaginal or anal intercourse,30 when he "uncovers" [the woman], they both become liable for execution, kerait, lashes, or stripes for rebellious conduct. Whether they were lying or standing,31 liability is established by the insertion of the corona.
[There is never any liability when] a man engages in forbidden relations without an erection, instead his organ was hanging loosely like the organ of the dead, e.g., one who was sick or a person with a congenital malady, i.e., he was born sexually inadequate. Even though he inserts his organ with his hand, he is not liable for kerait or lashes. Needless to say, he is not liable for execution. For this is not considered sexual intercourse.
This expression is used with regard to relations with a woman in the nidah state. From that instance, Yevamot 54a derives a connection to the entire Torah.
If, however, the man merely touches the entrance to the vaginal channel with his organ, he is not liable (see Beit Shmuel 20:3).
Or Sameach notes that Leviticus 18:23 explicitly mentions a woman standing while engaged in forbidden relates.
If one has bodily contact or petting with one of the arayot (people with whom sexual relations are forbidden), or he hugged and had genital contact (literally kissing; c.f. Yevamot 55b) and benefited from nearness of the flesh, he receives (biblically sanctioned) lashes, and in suspected of having relations with arayot. Note: if he had intercourse with her, whether vaginally or anally, as soon as he begins intercourse, which is defined as insertion of the glans, he is liable for death or excision. And the witnesses do not need to observe closely similar to insertion of the eyeliner in the tube, rather as soon as the witnesses see them intertwined with one another in the manner of adulterers, they are punishable by death (Tur), and the women is forbidden to her husband (Nimukei Yosef, Yevamot chapter 2). An adult woman who had relations with a minor male under the age of 9 years, is not liable for death at his hand (Tur). It appears to me that she is similarly not forbidden to her husband. And therefore the Rabbi, author of the Turim code wrote many laws defining when people are liable for death or not. The modern day repercussions of these laws (when we no longer administer capital punishment) is that they define which relations cause a woman to become forbidden to her husband, and see further. A female less than 3 years old, her intercourse is not considered as intercourse, and her virginity returns (Hagahot Yevamot). Even if the years were intercalated (e.g she had intercourse at 37 months old in a leap year) her virginity returns (words of the Rav, based on Yerushalmi Ketuboth chapter 1).
the Auruch Hasulchan 20.16-17 brings that she is forbidden from her husband even if the penis was not erect , and that even if only the tip was inserted she might become forbidden to her husband