You’re asking a lot of questions here and most of them have absolutely nothing to do with having a romantic relationship with your working partner.
Some things first:
1) The relationship between a battle couple and any platonic working partnership are not really any different in most cases except that they share a romantic relationship.
2) You don’t need a functional or professional partnership or partnership at all to fight in a group or gang up on an individual.
3) Fraternization just as often falls into casual sex as it does a romantic relationship, if not more often.
4) Almost none of what you’re asking has to do with romance.
Falling in love on the battlefield happens, it happens a lot. Combat is a high stress environment and people are people. Just because something isn’t a good idea or is unprofessional doesn’t mean it won’t happen, it just means you’ve got an added benefit of complications.
Some people can handle romantic relationships with an SO who also engages in combat, even one who engages in combat with them. Those are the ones who can compartmentalize between being on the battlefield and being off it. However, if they can’t (there is a very good possibility that they can’t) then it becomes a real problem. When they can’t handle the stress or the distraction, if they can’t put the romance aside, then their relationship puts everyone at risk, including their mission.
When you’re fighting, especially with a goal in mind, one person’s life cannot be more important than the mission.
It takes a significant amount of trust for a battle couple to function because their romantic partner cannot afford to jump in and save them when things start going sideways. Both participants need to be the kind of people that when the choice is between their partner or the mission, they choose the mission.
This concept is one that’s very difficult to grasp if you’re setting out to write a romance, because most of the normal steps you’d take to fulfill that romance will leave the battle couple hamstrung and unable to function. You can’t have the guy or girl jumping in to save their guy or girl when it looks like they’re about to die, they have to trust their partner to save themselves.
That is hard.
This is a very difficult state to handle emotionally. Imagine, you are at risk of losing your loved one at all times and you can’t do a damn thing about it. You can’t obsess or brood over it, because you can’t afford that kind of distraction. Whether they’re right in front of you or on a battlefield somewhere else, you can’t think about it. You’ve got to focus on keeping yourself alive, because that keeps everyone else alive, and by doing what you can you help to ensure the survival of both your loved one and your team. You’ve got to do your job, even when you’re about to lose everything you ever gave a damn about and its within your power to stop it.
A true battle couple is one who exists in complete equality, trust, and partnership with their significant other on the battlefield. They keep a cool head and a cool heart while in the midst of gut wrenching emotional turmoil. They don’t baby, they don’t hover, they don’t keep a careful eye on, and they don’t obsess until the fighting’s over. They don’t sacrifice their own life or their own body to keep their lover from getting injured. They don’t break position.
If they do any of the above, they will both die and so will anyone who is relying on them. If you are writing characters where the relationship is more important than the mission, more important than the team, more important than surviving the fight in front them then you have, narratively speaking, a serious problem.
This is not a bad one to have in a story or an unrealistic one in life, romantic relationships on the battlefield are built around this concept, but it does need to be addressed. If its not, tragedy strikes.
If you’re writing a battle couple, you need two characters who when faced with the choice between saving their loved one and stopping the bomb from blowing up downtown Manhattan, they pick the bomb.
And, in fiction, that’s not normally what love is.
It also has to be both of them, they both need this very specific outlook to function while in combat together. If one has it, but the other doesn’t then tragedy strikes. If neither have it, tragedy strikes. They need to be on the same page.
The reason why the military and other combat groups prohibit fraternization is because romantic relationships inevitably fuck everything up. If they can handle it, great. However, the all to likely outcome, for either one or both parties involved, is they can’t.
They’ll do it anyway though, because people are people.
When you engage in violence, that violence and training separates you from the general population. You’ve been through experiences that most people cannot comprehend or relate to and that makes maintaining relationships difficult. There’s a lot to be said for being in a relationship with someone of similar background, who can empathize with your experiences, who has been through what you’ve been through. You don’t need to look much further than the rate of divorce among the FBI or CIA to understand just how difficult maintaining a relationship in an incredibly stressful environment is.
As humans, we crave having a partner we can relate to. With whom we can share our secrets. Who won’t judge us for the terrible things we’ve done. When you have to rely on each other for survival, attraction, desire, even love becomes easy. It’s often a false sense of connection built on desperation, one which if born inside the environment won’t function outside of it, but that doesn’t mean it feels any less real.
When you might die tomorrow, sometimes you just want to feel something, anything at all, and that’s where the causal sex comes in.
In mixed gender units, casual sex is really common. Not romantic relationships, mind. It’s just sex, and it doesn’t go any further than that. It’s desperation, it is all about sensation, and a reminder for the participants that they are alive.
When dealing with these types of relationships in your fiction, its important to remember that the emotional component is neither needed nor wanted. They’re not looking for comfort. They’re looking for sensation, to feel something before they (potentially) die.
Because the author controls everything in their fictional world, it can often become difficult to remember and insert qualities like the random chance of dealing with the unknown. We’ve often got characters that are necessary to the plot, who become identified as “safe”, and behave differently because they know they’re going to live through the fight or battle to get to the end of the story.
It becomes important to learn to live in the moment. To live in the twilight hour on the night before a battle, to be unsure, when the character doesn’t know what will happen next. If you don’t then there is a whole array of human emotions, experiences, and terrible choices that you’ll never touch on in your fiction.
If you don’t, you’ll be all the poorer for it.
The Two on One Battle: Real.
You don’t need to be in a relationship, or even particularly well-trained, to accomplish this. Two versus one happens a lot and the pair off usually wins because eight limbs trumps four. One person locks up the individual, the other circles and attacks on vectors they can’t defend from. We’re social animals. Our natural instincts will help us more when we’re fighting in a group as opposed to fighting alone.
1 v Group is a bad situation to be in if you’re the one, and it doesn’t matter how well trained you are. Numbers will kill you.
Part of the reason why you see single characters fighting groups in movies and other fiction is to establish that they’re great fighters. The problem is that this has become so widespread that we now think fighting a group is easier than fighting a single, skilled individual. This is untrue. The group will kill you because the individuals within the group can move onto vectors that cannot be defended.
What your describing in your question in a battle between three people in a two on one is normal behavior, its standard tactics. However, you’re also demonstrating the exact kind of behavior for why two people engaged in a romantic relationship should not be on the battlefield together.
If you’re ever sitting there and wondering if something that is a basic and bog standard tactic is now, suddenly, too dangerous because your characters are dating then that is the exact problem.
Things that are normal suddenly become too risky, and the focus transitions to preserving their lover’s life rather than making use of their significant advantage over their enemy.
That is the exact kind of thinking which will cost them their lives, and for no benefit at all.