Edit
When at least one tentacle is inserted into a character's vagina and/or anus to mimic the act of sex.
As per topic #13661, this is considered to be a third category of sexual activity aside from sex (with a penis) and object insertion (using a foreign object in a sexual manner), since tentacles are not considered immobile objects, yet are not considered penises either. Artists often portray tentacles performing the same function as a penis, hence the need for this third category. The rules for vaginal and anal would still apply here.
Don't use this tag if the tentacle is just about to be inserted, but hasn't penetrated yet. Use imminent sex/imminent rape and imminent vaginal/imminent anal instead.
EXCEPTION: If there are tentacles present in a sexual image, but they're not the actual subject of penetration. Do a sex tentacles -tentacle_sex search to find such images.
Examples:
post #2509701 (penis = sex)
post #817291
post #2171699 (tentacled monster girl having sex)
post #807415 (example of both sex and tentacle sex)
See also
This tag implicates tentacles (learn more).