Prompt: Jeeves/Bertie, compersion (i.e., the feeling of happiness and/or arousal one gets when one's partner is with other partners. Basically the opposite of jealousy.)
There are various ways this could go: Could be pre-relationship or open relationship. One could accidentally walk in on the other in flagrante delicto. Bertie could come asking Jeeves for sex advice regarding his new boyfriend or girlfriend. The important part is, one gets laid and the other is turned on by it.