I'm looking for a manga where the only gender in that world is male.

You mean a world officially without women? Don't see any 😐 But I can't remember one single girl in Kirai Ja Nai Kedo either (apart from some faceless and nameless fodder). Would that fit, somehow?

It's the Art of Time, it's the Art of Life--
Of the player and the craftsman,
Of the writer of songs, of the creator of love,
Of those who will follow, of those who led,
There's no other way in the world ahead ♥

There's actually a tag for this, but the only yaoi on it is Ai no Kusabi (along with Marginal, if you're counting shounen-ai). Huh.

Otherwise, there's a lot of yaoi with only males, but females still exist. They just don't show up at all.

I thought they all fell under that category. 🤣 I think you would fine that the opposite is a lot more rare.

A popular one where it really does look like no women exist is Love Mode.

Unless you actually do mean a world where the population is only male. O_O

[img]http://28.media.tumblr.com/tumblr_lq2hczRc5d1r1rvcno1_500.gif[/img]
Monkeys, always at your service.

Quote from NightSwan

I have to thank you again. It's been a while since I've had this much d**k on my computer. :'3

Nightmare Fortress - Majin no Jousai

Gakuen Heaven
Sukisho
Princess Princess

LOL these are pretty in common and i couldn't think any and all of the said titles were adapted to anime (LOL in case you haven't view 'em yet xD)

 わ た し は 、 あ な た を 愛 し て い ま す   
W a t a s h i  w a ,  a n a t a   o  a i s h i t e   i  m a s u

Honoka na Koi no Danpen wo and the sequel Goshintou and every manga from DUO BRAND.