That's the very argument, though - sure, Serena coerced (and monitored, ick) the first instance, but June and Nick both willingly and consentingly continued the relationship - apparently covertly. The legalities may say otherwise, but morally and ethically, what happened every time after the first was consentual and had nothing to do with Serena - it was used by June (in my view, anyhow) as a way to rebel, to get a small measure of human comfort, as a way to take back her own agency. We can "well, legally..." things to death, but what matters is the intent and how June felt about the subsequent encounters, because as we all know, personal experience is personal and what one person would consider to be traumatic/romantic/scary/sweet/assault/offensive/funny/creepy etc is for THEM to call that thing, not for us to TELL THEM how they should feel about it. The fact that June herself says in the tapes to Luke that the baby was concieved in love should say everything - she wanted to ensure that Luke knew that the child wasn't a result of rape in her eyes.