The answer by wetcircuit is already a very good frame-challenge for how this whole "win back the girl" plot is one that can easily turn out very bad if not handled properly, because you risk to dehumanize the female character into nothing but a trophy for males to fight over. But I would like to address the actual question:

Assuming you indeed want to write a "win back the girl" story nevertheless, how graphic should the sexuality between girl and antagonist be described?

Keep in mind that the reader is supposed to sympathize with the protagonist. So whatever you write should make the reader feel the same thing the protagonist feels. Except for a small minority of people who have a cuckolding fetish, the thought of a person you love being sexually intimate with someone else is rather off-putting. So if you try to write the sex between girl and antagonist in a titillating way, that will probably backfire. Either the sex is good, and you create cognitive dissonance in your readers. Or the sex is bad, and you have another plothole to pave over regarding the motivation of why girl stays attached to the "evil" antagonist.

What you could instead do is describe from the protagonists point of view how he imagines their love-life and how he is disgusted and hurt by that thought. And then leave it to the readers' imagination how their love-life actually looks.

Should they (not in the imagination of the protagonist but in the actual reality of the narrative) have a sexual relationship? That's something you should leave up to the characters. Is the antagonist sexually interested in the girl? Is the girl sexually interested in the antagonist? Do they have the opportunity to spend enough time with each other to get over their personal "knowing a person well enough to have sex with them" thresholds? All of that depends on how you characterize them.

The answer by wetcircuit is already a very good frame-challenge for how this whole "win back the girl" plot is one that can easily turn out very bad if not handled properly, because you risk to dehumanize the female character into nothing but a trophy for males to fight over. But I would like to address the actual question:

Assuming you indeed want to write a "win back the girl" story, how graphic should the sexuality between girl and antagonist be described?

Keep in mind that the reader is supposed to sympathize with the protagonist. So whatever you write should make the reader feel the same thing the protagonist feels. Except for a small minority of people who have a cuckolding fetish, the thought of a person you love being sexually intimate with someone else is rather off-putting. So if you try to write a detailed sex scene between girl and antagonist from their perspective, that will probably backfire. Either the sex is good, and you create cognitive dissonance in your readers while they read about it. Or the sex is bad, and you have another plothole to pave over regarding the motivation of why girl stays attached to the "evil" antagonist.

What you could instead do is describe from the protagonists point of view how he imagines their sex-life and how he is disgusted and hurt by that thought. And then leave it to the readers' imagination how their sex-life actually looks.

Should they (not in the imagination of the protagonist but in the actual reality of the narrative) actually have a sexual relationship? That's something you should leave up to the characters. Is the antagonist sexually interested in the girl? Is the girl sexually interested in the antagonist? Do they have the opportunity to spend enough time with each other to get over their personal "knowing a person well enough to have sex with them" thresholds? All of that depends on how you characterize them.

The answer by wetcircuit is already a very good frame-challenge for how this whole "win back the girl" plot is one that can easily turn out very sexist. But I would like to address the actual question:

Assuming you indeed do want to write a "win back the girl" story, how graphic should the sexuality between girl and antagonist be described?

Keep in mind that the reader is supposed to sympathize with the protagonist. So whatever you write should make the reader feel the same thing the protagonist feels. Except for a small minority of people who have a cuckolding fetish, the thought of a person you love being sexually intimate with someone else is rather off-putting. So if you try to write the sex between girl and antagonist in a titillating way, that will probably backfire. Either the sex is good, and you create cognitive dissonance in your readers. Or the sex is bad, and you have another plothole regarding the motivation of why girl stays attached to the antagonist.

What you could instead do is describe from the protagonists point of view how he imagines their love-life and how he is disgusted and hurt by that thought. And then leave it to the readers' imagination how their love-life actually looks.

Should they (not in the imagination of the protagonist but in the actual reality of the narrative) have a sexual relationship? That's something you should leave up to the characters. Is the antagonist sexually interested in the girl? Is the girl sexually interested in the antagonist? Do they have the opportunity to spend enough time with each other to get over their personal "knowing each other well enough to have sex" thresholds? All of that depends on how you characterize them.

The answer by wetcircuit is already a very good frame-challenge for how this whole "win back the girl" plot is one that can easily turn out very bad if not handled properly, because you risk to dehumanize the female character into nothing but a trophy for males to fight over. But I would like to address the actual question:

Assuming you indeed want to write a "win back the girl" story nevertheless, how graphic should the sexuality between girl and antagonist be described?

Keep in mind that the reader is supposed to sympathize with the protagonist. So whatever you write should make the reader feel the same thing the protagonist feels. Except for a small minority of people who have a cuckolding fetish, the thought of a person you love being sexually intimate with someone else is rather off-putting. So if you try to write the sex between girl and antagonist in a titillating way, that will probably backfire. Either the sex is good, and you create cognitive dissonance in your readers. Or the sex is bad, and you have another plothole to pave over regarding the motivation of why girl stays attached to the "evil" antagonist.

What you could instead do is describe from the protagonists point of view how he imagines their love-life and how he is disgusted and hurt by that thought. And then leave it to the readers' imagination how their love-life actually looks.

Should they (not in the imagination of the protagonist but in the actual reality of the narrative) have a sexual relationship? That's something you should leave up to the characters. Is the antagonist sexually interested in the girl? Is the girl sexually interested in the antagonist? Do they have the opportunity to spend enough time with each other to get over their personal "knowing a person well enough to have sex with them" thresholds? All of that depends on how you characterize them.