The scene was called "disgusting and damaging" by one viewer, who urged TVNZ to screen the show in a later timeslot.
TVNZ has defended the show, which airs three times a week in a 7.30pm timeslot, with an "uncut" episode airing at 9.30pm on Fridays, saying it "reflected the realities of dating in a Tinder age".
Nixon said she was forced to pen her open letter after her daughters, aged eight and six, saw advertising for the show during the early evening.
She called on TVNZ to scrap Heartbreak Island completely because it depicted an unrealistic view of love and dating.
"Heartbreak Island advertising has been intruding into our early evening TV-watching and I'm now having to educate (my daughters) why they will never date or meet a boy in this way - that this isn't normal," Nixon wrote.
"We are human beings, not sexual objects to be used and abused."
She warned that unless the show was moved to a later timeslot, and advertising was limited to adults-only timeslots, she didn't want to watch any of TVNZ's channels.
"In a culture rife with mental health, suicide and a host of other troubles, you have created one more to help torment the young people of today, because this show is nothing but trouble," she said.
A TVNZ spokesperson said Heartbreak Island promos had only been scheduled to play during television programmes with appropriate corresponding classifications.
"This means that PGR promos are being played in PGR programming. Promos have not been scheduled in children's programming - nor will they be," the spokesperson said.
"Heartbreak Island is a bold new show, while it might not be for Jane, it is for a number of our viewers."
Read Nixon's full open letter to TVNZ:
"I'm a mum to girls, eight and six years old. Heartbreak Island advertising (and that's just the advertising) has been intruding into our early evening TV-watching and I'm now having to educate them why they will never date or meet a boy in this way - that this isn't normal. TVNZ you have lowered the bar to such disgusting standards, I feel like I don't even want to watch your channels anymore. In a culture rife with mental health, suicide and a host of other troubles, you have created one more to help torment the young people of today, because this show is nothing but trouble. Was it your intention to create a platform where these contestants are treated like pieces of fleshy meat meant nothing more than for sex, shame and money? Do you mean to normalise Tinder-like dating for the most vulnerable to watch? To send not-so-subtle messages to any person young, old or in-between about how dating and love might unfold? It's stupidity beyond what I thought a responsible broadcaster could be capable of. We are human beings, not sexual objects to be used and abused. You have now made the further step into setting up a TV culture that can only be described as harmful and nothing good can come of it. I applaud everyone who protests this show and I hope you throw it away into the nearby ocean it's filmed by. In my view your professional brand is well-tarnished now but there's still time to reconcile the situation. The people are speaking. Are you listening?"