i dont think you really do thank your partner after sex, or foreplay or anything. reason being, it just awkward, and it puts your partner in a situation where he/she doesnt know what to say.
example, i come up to my partner and say 'i love you'. my partner returns to me and says 'thank you'.
if anything, just say something like 'that was amazing!' or 'i love what you did there andhere' and stuff
just my two cents
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