I'm new and starting out. But karet sounds serious. Is it just the standard intimate relationship that gives karet or does unnatural and other practices as well. What about kissing or other non penetrative relationship. One rabbi told me even touching a women who is nidah gives karet.
What is Chayav Karet?
Starting Biah, which is called heara (i.e. at least penetration of the glans), is defined as biah (intercourse) in Arayot (prohibited sexual relationship) rules, whether vaginal or anal, and is called by Tora mishkeve Isha (Leviticus 20, 13) ("woman's lying")). We will partially demonstrate this further.
Torah told about prohibition and about punishment for who break the prohibition intentionally.
The verse which prohibits is (Leviticus 18, 19)
"Also you shall not approach a woman to uncover her nakedness, as long as she is put apart for her menstrual uncleanness.".
"approach to uncover her nakedness is translated concerning nidda, following a part of Rishonim (Ramban at least) as regarding sexual intercourse only.
The verse which communicates the punishment (the Karet) is in Leviticus18, 29
For whoever shall commit any of these abominations, the souls who commit them shall be cut off from among their people.
The Karet punishment is according to everybody for sexual intercourse only. This last point is written in Misnna.
The mishna said this in Kretot 1, 1 in the list of all chiuve Kretot that for nidda intercourse is the problem:
שְׁלשִׁים וָשֵׁשׁ כְּרֵתוֹת בַּתּוֹרָה. הַבָּא עַל ...... וְעַל הַנִּדָּה
There are in the torah thirty-six [transgressions which are punishable with] extinction: when one has intercourse ... with a menstruous woman;
See a summary of the Halacha, from Sefer Chinuch 207:
ונוהג אסור זה בכל מקום ובכל זמן. ועובר עליה ושכב את הנדה במזיד, מכיון שהערה בה חיב כרת, בשוגג מביא חטאת קבועה. ובן הנדה אינו נקרא ממזר אלא ולד פגום. ובלאו דלא יבא ממזר נבאר זה בעזרת השם בסדר כי תצא
This mitsva is relevant everytime. One who break this law intentionally and penetrate (at least with the glans) is punished by Karet...
Touching a woman nidda doesn't give Karet as the Chinuch summarized, intercourse only is concerned by karet.
 Gemara Sanhedrin 55a.
R'Nahman, son of R'Hisda stated in an exposition: In the case of a woman, there are two modes of intimacy,