I think Wendy's sexuality and her relationship really matters in this series. First, they are lesbians and lesbianism is always good
Second, the relationship may be a bit drawn out I agree but it's really important for the series above all when taking into account that sexual deviancy (as viewed in the 70-80s) and sexual gratification are an integral parts of the serial killers profiling. So sexuality as a whole matters but more importantly so does homosexuality here. Wayne was being pushed with this angle and actually was in the closet, Wendy connected with one of the serial killers with her experience (how she kinda was groomed by her old professor), she also cornered another closeted one/homophobic etc.
Then you have serial killers like Dahmer, Gacy and other famous ones that are homosexuals so I think in the future seasons we might see Wendy conflicted during some interviews with bi/gay serial killers and if Bill and Holden push that angle on them, that could potentially create tension in the team. She's portrayed as a strong, cold woman in a mostly male work environment but we started seeing some cracks in that armor this season with her gf so with Bill's situation, that could be something that could played on too. He confided in her about his kid so she may end up doing the same about her sexuality.
Plus in that time period, homosexuality was seen as a deviancy (Gregg's remark that Wendy corrected with the DSM stuff) so considering how others sexual deviancies (such as pedophilia (Wayne), transvestism (BTK), necrophilia (when Bill talked about the serial killer that was fukkin corpse's heads)) lead to profiling potential serial killers, I think it's a good facet of hers to have there. She knows she has to be in the closet for fear of being judged in her work environment that is not gay-friendly at all. So the whole relationship with the hippy gf was interesting as it challenged her own closet situation. Though ultimately, it just reinforced her ways as the gf was basically lying to herself and not that "open" in the first place.