If you re-wath that episode Serena literally is basically just the pimp who sets it up. She asks both of them to be willing participatants and neither of them seems to have a lot pushback. June's only argument against it is that it's forbidden. At no point does it show that they're threatened or intimated into doing it.

Sure, Serena is the woman of the house but she has little to no power in Gilead. If anything I'd say Nick and June would have had the upper hand and could have gotten her into a crap ton of trouble.

Let's not forget, Nick was an Eye at the time. If he really did not want to have sex with June, he easily could have done something about it.

Realistically, Nick had the most power at the time so I'm politely going to have disagree with your analysis of it.