Do you take on a partners bad karma when you have sex with them? This all depends.
In the Brihadarankaya Upanishad it says “The man who practices sexual intercourse while knowing the formula of Karma and it’s action takes to himself the accumulated good Karma of the woman; he who makes love without knowing such a formula stands the risk of losing his accumulated good karma to her.”
The level of awareness of each partner as well as their intentions determine how a sexual energy exchange will manifest. If there is loving compassion and giving, the sexual act will be enriching for both partners. If there is selfishness, then a negative karmic exchange will occur.
Tantric texts confer sexual initiations and protective rituals to help establish the purity of the sexual act and ensure a beneficial and empowering exchange for both partners, who strive to attain the spiritual ideals of union.