In reality, having an extramarital relationship has been a partial or total motive or even just a precursor for many of the more infamous parents who murdered their children (thinking Susan Smith, Diane Downs, Scott Peterson). So in that sense if TH is an actual suspect, then ANY extramarital relationship would be important to identify.
Please though, I don't think a lesbian relationship should be short-handed to "sexual" - that's not necessarily what a romantic interest in another person is all about. I am a woman who romantically likes men, but I'd really bristle at someone characterizing that as "sex". Sex is part of a romantic relationship, but the act of sex isn't what defines someone's romantic leaning. IMO, it doesn't matter if Dede and Terri had a physical relationship - what matters is if Terri had any relationship at all that created a motive for abduction/murder, or that showed her state of mind at the time Kyron vanished. Whether they were doing the deed or not, who cares. Whether one or both of them considered themselves romantic DOES matter, IMO.