So as far as the "Sersi and Dane are only friends in Eternals" slander coming from a certain person who likes to spread lies, including about yours truly:
Dane is in eight scenes (yes, I counted) and about ten to fifteen minutes of screentime in Eternals. It is definitely written as a romantic relationship. Among the things we observe are:
- Dane taking over for Sersi's natural history lecture and making plans for their date that night, and her students ooh-ing and ah-ing over it
- some dancing at a bar and a birthday celebration where Sersi gives Dane a ring with his family crest on it (indicating she took some time to pick out something meaningful to him, like a girlfriend would)
- Dane and Sersi embracing in a stairwell, Dane asking about her ex, Dane asking her to move in with him (meaning they are almost certainly having sex), and a make-out session
-Dane, Sprite, and Sersi walking on the streets, where Sprite and Sersi discuss why she can't move in with Dane (supposedly because she has to take care of Sprite)
-The Deviants attacking, Ikaris's re-emergence, and Dane and Ikaris having a face-off when Dane realizes who he is
- Dane and Sersi again discussing her reluctance to move in with him, a long conversation about Ikaris and the Deviants, and Sersi telling him she has to go, at which point they embrace goodbye
- Sersi calling Dane and advising him to make amends with his family: Ikaris asks if they are breaking up, and Sersi denies it. He then says he misses her and seems about to make a play for her, but is snatched by Deviants, and the topic is dropped
- Dane meeting up with Sersi again after Ikaris removes himself from the narrative, hopefully as painfully as possible
- A love confession from Dane, who promises to love Sersi no matter who or what she is, which is unfortunately interrupted by Sersi getting kidnapped by Arishem right as he's about to spill his family secret
Dates, bar dancing, gift giving, making out in a stairwell, multiple offers of cohabitation, long discussions about the ex, knowledge of family tensions, a confirmation of a romantic relationship, the ex only making romantic overtures when he knows he has competition, a love confession....in no context can this be considered anything other than romantic.
If you don't like it and you consider the epitome of romance to be hesitating to kill your estranged wife after you have viciously and brutally killed or attacked two other female characters who are also family members, go ahead. But don't lie about what actually happened.