TL;DR: Primitive Boyfriend was... weirdly good. Should you read it? Depends how you feel about monkeys.

L;DR:
I thought I'd seen everything in the shoujo manga world after that one about the girl falling in love with a lizard (Tokage Ouji)... but Primitive Boyfriend just had to strut in and blow that out of the water. The concept? Bizarre. Wild. Weird. The execution? Cringe-inducing. But... damn, it kind of works?

Protagonist Mito returns to monkey, quite literally, as she is sent a few million years in the past by a delusional goddess to a time where humans were big buff monkey-dudes. And one of them is her soulmate, apparently. Of course Miss-Not-Like-Other-Girls falls head over heels for him. It's quite frankly weird, but hell, this monkey is a lot nicer than her other options, so maybe I don't blame her.

What follows is a series of weirdly educational isekai-ing back and forth through time periods. There were some sweet parts, some cute parts, and some what-the-actual-hell-am-I-reading parts. For only 13 chapters, it crams a LOT of substance in that made me honestly enjoy my time. And the ending... yeah, that was pretty cute.

Art is nice enough and the manga becomes weirdly atmospheric at times. The characters aren't really fleshed out that much, but I didn't expect too much from a 3-volume manga. I also thought that the explored theme of soulmates was interesting. And that's probably the most I can say about it.

Should you read it? That's a tough question. I honestly think you should just dive in blind. Laugh at it, don't take it too seriously, and enjoy it brainlessly.

And if you end up seeing the physical volumes on my manga shelf, mind ya own business.