Okay. No jokes.

First congratulations. It is an honor that this couple has decided they can trust you in this way and that you will honor their trust and privacy. That's big.

I have not done this, but I do sex therapy with individuals and couples and I understand their desire and your difficulty. They are trusting you to take this seriously and I apologize for the children on this forum.

There is nothing especially difficult about this. I have shot a lot of nudes with a single off-camera flash and a Gary Fong Lightsphere. More recently, I have moved to a Light Genius as a modifier--mostly because it fits in my bag better. Either one will give some modeling and reduce the harshness. Straight bounce flash would also probably work, but bounce depends on the color of wall/ceiling paint and distance, etc. A bracket that moves the flash about 4-6 inches to the side and 4-8 inches above the lens will give a little direction to the light. It would be best if they stay further down on the bed to keep any shadows off the walls.

Having said all this about flash, I would try to set up enough steady lights that I would not need flash. Even some cheap clip-on lamps with higher-power cfls aimed at the ceiling. The main reason would be to overcome any lighting from windows or room lights. I would push the ISO as high as needed to avoid the flash. (My a6300 will do well at 6400 and even good enough to 12800). A little noise will be irrelevant. I don't know your camera, mine will shoot in a quiet mode without making any sound. Anything to keep from interrupting them or making them conscious of the camera.

Treat this as their own personal sex fantasy with the emphasis on romance, not just raw sex. So not like most porno, but more like erotic photography, explicit and exposed, but not "raw."

Take a couple of test shots before they enter the room, as you won't get a second chance.

Have a private conversation with the couple a couple days ahead. Talk with them about what they want. I would want to go over the whole process with them. Share the shooting script that I have suggested below with any other shots that you might want to add. Get their reactions and any additions. If they make requests be sure that you get those shots. If you've come this far, you probably don't have any objections to anything they suggest but, if you do, be sure they know and then negotiate modifications. Just remember that this is their show and you are just a means to them getting their desires met.

Be clear with them that you will try to get everything for them but if you can't, you are not at fault. They can ask you afterward to shoot them acting that shot out.

Make at least a couple dozen different shots at different angles. I would shoot at least the following shots.
1. At least a couple of kisses, fully clothed
2. One or more shots of him removing her clothing, especially a shot of her in bra and skirt or bra and panties.
3. Get a shot of her removing his pants. Perhaps a shot of her fondling his penis or kissing it.
4. One or more shots of him removing her bra, with a good shot of her breast partly exposed but before the bra actually drops.
5. A shot of her bottom, nicely rounded, perhaps even showing her genitals.
5. A shot or two of him placing her on the bed, fully exposed.
6. The rest depends on their choice of sexual activity. They may spend some time simply kissing, or with him kissing her whole body. If he is a good lover, he may well spend a lot of time kissing her genitals, or the inside of her thighs and breasts. There is no way to know for sure.
7. Depending on their choice of position for actual intercourse, you should get a shot of him entering her.
8. If you are able and quick enough, you may be able to get the expressions on their faces as they orgasm.
9. Add anything else they want.

You are there at their request. You are not an intruder. Still, your very presence could detract from their pleasure and joy. You must be very careful not to interact with them but to let them do this on their own. You can't stop them to pose them. Make yourself as unintrusive as possible. If that means you miss something, perhaps they will be willing to simulate it afterward, just don't interfere during their activity.

On a personal preservation note: You will likely be hard pressed to avoid your own sexual reaction. So anything you can do to damp down that reaction beforehand will be helpful. You are like a gynecologist or urologist. You are not involved except to provide a technical service. It is what we call a "clinical setting."

Also for personal preservation, I would rewrite a photo release form so that it explicitly states that all the images are private and only available to the couple for any distribution, that you are specifically required to keep them private and not available to any other persons. They will almost certainly request this. Even be prepared for them to request you to delete any images from your system.