If you put out a lawn care book, I’m buying it. Autograph it for me? 
From the OP
Wanting privacy for showering: Not weird.
Casually walking in and using the facilities while someone is in the shower: Weird. I want privacy for that part, too!
Just my subjective two cents.
that was stated not because i didn’t read the OP but to put a context for the rest of the reply if that had been done as stated.
sometimes my expression is clear to me but not always the same to others.
I personally don’t need any privacy for showering, but don’t think the OP’s feelings and requests are strange or out of line. We have clear glass on our shower and I often remark to my wife how great she looks in there. If it bothered her, I’d stop.
I’m totally on your side; I value my privacy too, even though my partner and I have been together over 27 years. Shower space is private, period.
But if your BF has a urinary condition, like “urge incontinence,” that changes everything. When I have to pee, the need hits with no warning, and I have about 10 seconds before I start squirting out. That’s usually just enough time to make it into the bathroom, but getting outside would take too long. If this is the issue, please cut him some slack. And encourage him to see a urologist (I’m seeing one tomorrow).
I’ve got a medical problem, called interstitial cystitis, and I took a medication for five years, after having a small procedure on my bladder. I was doing pretty well for several years, then after I had a baby, it flared up with a vengeance, so I’m back on medication.
When I have to pee, it can be so damned uncomfortable, it might as well be pain, and sometimes it comes on suddenly. Before I had medication, I sometimes had episodes where I literally had to pee every five minutes, for like 30 or 40 minutes at a time. If you knew how bad it felt to have to wait under those circumstances, nevermind I’d just peed 10 minutes ago, I think you’d be willing to give up a minute of your privacy.
Honestly, I’m not even sure what it’s like for those people who pee just a few times a day. I can make it through a movie, I’ve been known to make it through a four how car ride, if I don’t drink anything, but it’s a good night if I only get up once, and I usually go about every two hours. Sometimes every hour.
If he starts doing this every single time you shower, maybe you guys need to put in another bathroom. But if it happens once every couple of months, can you assume that he really, really has to go? I mean, you’ve told him how much you hate to be interrupted, right? If you haven’t, then this is on you.
Next time he walks in on you try to act like you didn’t see him and start moaning his best friend’s name like your having your “alone time” in there.
Shutting the door to shower or pee is above my level of wanting privacy if it’s a SO. I’d only shut it then if she wasn’t comfortable. Likely I am weird. The issue isn’t whether something is weird or normal though. It’s finding a way to make negotiating the differences reasonable to both.
Even if that’s something like bothering to shut the door to not offend them. 
My family has always issued public alerts. “I’m about to take a shower; anybody need the bathroom?” Last chance to pee.
I don’t see any problem with wanting some time to yourself while you’re in the shower. I don’t see why any grown man can’t hold it for 15 minutes or find and alternative place to pee. I would find it really suspicious if my husband suddenly had to pee every single time I wanted to take a shower, especially if I had already warned him I was going into the shower. It seems to be less of peeing issue and more of a control issue. Why can’t he just honor your request and go before you get in?
A BF? ( boy friend? )
Monogamous relationship for how long…
If less than 2 months, dump him. 
I have a bathroom, she has a bathroom. Period.
I can’t believe I’m the first to suggest this, but maybe he finds it kind of erotic to come in the bathroom while you’re in the shower. Hell, maybe his peeing in front of you plays into it.
Yes! I’ll share a lot of things with my DH; we’ve been together for 30 years. But Not the Bathroom!
Not really related, but the OP reminded me of this:
What happens when a boyfriend ignores his girlfriend’s request for privacy
I don’t think you’re being unreasonable, and the thing is, if it’s important to you he should respect that. I am the same way. My bathroom time is my private time, so do not bother me.
Awesome post from the comments: “and that’s how you change privacy settings with your partner.”
My take on wanting some alone time, and using the shower for that, is that it’s also nice to have the small space be all steamy and cozy and smelling nice from the soap and shampoo. If someone comes in and pees - the toilet can’t be flushed - so now there’s not only the cold breeze from the door being opened and possibly left open for the duration of a piss, there’s also a piss smell emanating from the toilet after the shower’s done. No thanks, jerkyface!
The OP’s request is beyond reasonable. I think she should take an hour long bath with the door locked.
Do you have a shower curtain or not?
I don’t see what the big deal is about knocking on the door and popping in and out to take a leak and wash hands. No lingering.