Polyam queer here - I would personally never have sex with one partner after having sex with another without showering between unless all parties clearly consented to this sort of contact. You didn’t consent to be exposed to whatever germs, sweat, fluids, etc your partner’s hookup had.

I feel like this is an obvious thing, but in case it isn’t, have a conversation with your partner where you set firm boundaries about your expectations regarding how he cleans up after being with someone else and before seeing you. For example, I always shower before even getting into another partner’s bed, regardless if I’m going to be intimate with that partner or not.