You gotta lean into it.

You don't scrub their back and otherwise just stand/sit there.

You bathe your partner while they bathe you. Yes, including the "dirty" bits. Yeah, you may need to go back and get spots that get missed because you can only get so good when going by feel until you have a lot of practice, but it isn't that difficult; I was giving baths to elderly patients when I was 17, and teenagers are all idiots (I was no exception).

Seriously, just lather up the scrubber, washcloth, or whatever y'all use, and wash them. ENJOY that shit! You're rubbing your sexy, wet, and loving partner. You very to touch every inch of them, as you scrub and rinse. For delicate places, you use your hands and gentle pressure (unless they like a harder scrubbing, but soap and genitalia isn't a good combination. Yes, really, I washed people for a living, their skin health was my responsibility. You don't need soap on your junk to get it clean, and most soap is too harsh for genital use).

And, yes, that also means you get in that crack and wash their butthole. Just get in there and have some fun with it!

If you're with someone, and you can't have fun when you shower together, there's something fundamentally broken in the relationship that needs working on. It's about intimacy, spending time taking care of each other, and enjoying each other's bodies. It doesn't have to be sexual, though there's nothing wrong with it being so.

The problem with bathing together is usually that most showers aren't built where both parties can be under the water at the same time. This can leave one person cold. But, if you're touching and scrubbing the person that isn't under the water, theft aren't likely to get very cold. And they possibly won't even notice if they did.

Even if it's every day, or you're just doing a fast shower, it's entirely possible to make it work and not be an impediment to getting clean and getting out.