TOS!Kirk ends up on the New Enterprise and meets AOS!Spock (or vice-versa). When this Spock reacts to his flirting, Kirk senses that he is more open with his feelings (and his sex life), so he immediately tries to seduce him.

-no Mirror!versions, please!

-if there's sex: bottom!Spock

Bonus:

-one of them has to explain the bruises later

-OR: They get caught in the act