Shockingly I was typing lots of variations of playboy becomes homo into the forums but I couldn't find a topic so excuse me if there are already loads cuz it seems like the sort of thing that would have been asked before.
Anyway:
As the title suggests I want to read a manga where the main character is a dude who is a total player with women and goes from one woman to the next and then meets somebody who completely changes his orientation.
I know there are a lot of them out there but I've forgotten them all asides from the one I just read:
Tsuyogari
And one where a dude is a host and he gets a new neighbour and likes his cologne blah blah.
I do have some specifications though:
Would like to read one where the story doesn't progress too fast - in other words oops he sleeps with a guy and suddenly realises he's a gay!
And no tragedies please.

Usagi Otoko Tora Otoko, perhaps?

Katsuai Monster.
Mimitabu no Riyuu.
Wagamama na Jewel - Chapter 4.

I know you've already read it but just in case someone else wants to know, the story about the host and the new neighbour is the side story in Abiru Junjou.

Ubawareru Koto Marugoto Zenbu
Overdose
50x50 (AMAAAAZING and funny, don't miss it!)
Ore ni Koi Shite Dousunda
Wild and Strawberry
2nd & 3rd stories from The Door to the Closed Mind
Seven Days (kind of)
Aishuu
My Servant and I (although it could be that the 'straight' player was gay to begin with...!)
The Carp on the Chopping Block Jumps Twice
Wonderful Days?
First Love (Sweet VS Home) (only the one-shot with that title)
Leopard Hakusho (a straight 'host' turns gay)
Tsugou no Ii Kimi
Waltz with a Black Cat
Prince Charming

Thanks everyone for your awesome replies. Will be checking a load of those out - shall be good reading in between revision 😛

I think that this meme is more common that not. My favorite is When a Man Loves a Man, about male hosts. Two guys 'turn gay' for Takaaki, and another for one of Takaaki's suitors. One of them insists to the end that he is only that way for Takaaki, though.

The Cornered Mouse Dreams of Cheese - the prequel to The Carp on the Chopping Block Jumps Twice... though... I feel like you've read it already since tartufo recommended the sequel. 🤨