something like Boku no Ano ko where the two characters are engaged since they were kids because of parents and the characters don't know the other character is the same gender and they had always loved them. At first one or they both mind they were the same gender... looking for yaoi or shounen-ai

All yaoi/yuri related topics goes in the their respective forums, even the I'm looking for's.

I can only think of this one
Full Moon ni Sasayaite

Only one I can recommend thats vaguely similar is Love stage!!!

Did a search and found:
Hanayome-kun and its sequel Hanamuko-san
Koi wa maiorita
Shitsuji game
Yome ni konai ka

Otherwise one of the mangas in either of these searches might do:
Yaoi+arranged marriage
Shounen ai+arranged marriage

Something exactly like that is difficult to find... not sure there is another manga with exactly the same themes as the one you mentioned!

Similar manga:

1. Hanayome-kun by Hoshino Lily
2. Koi wa Maiorita by Ajimine Sakufu
3. Kimi wo Metoraba by Dr Ten
4. Oyome ni Oideyo! by Kotobuki Tarako

You can also try using the genre search and category/tag filter to find more closely related stories...

This is a really good one! Totally cute!

Unmei ni Kiss

It's not exactly what you're looking for as the main characters haven't been engaged since birth (and one already knows that both are male), but you could try:

"Fragile but Strong" from Wagamama na Jewel by Sawauchi Sachiyo