I know the manga is really making it look that way, but if you read the novel translations at the end of the chapters you can see that there's no way he and the girl could ever be in a genuine relationship. Also, I don't think you intended it to be this way, but your comment is implying that sexuality is based solely on having sex. Sex and sexual orientation are two different things. For example, let's assume you're straight: Do you think you'd be able to have a romantic relationship with someone of the same gender as long as you didn't have sex? Do you think that you'd genuinely be able to develop feelings for that person? Most people would say no. And it's likely that the reason for saying no has nothing to do with the fact that you can't have sex, it's the fact that most people simply can't harbor romantic feelings for someone outside their sexual orientation. I know I didn't do a very good job of getting my point across but I hope you get the idea.
What you're describing here: "i feel like if you like someone you would automatically developed sexual feelings for then without minding the gender" would actually fall under "pansexual" or even "demisexual", which are both sexualities. And just like how not everyone is gay or straight, not everyone is pansexual or demisexual. If you find yourself able to relate to what you said about being able to become sexually attracted to someone after developing romantic feelings for them, then you may be pan or demi :)

I dunno why i don't want him to end up with the girl ╮( ̄▽ ̄)╭ i know they can have a relationship without sex but it just doesn't feel right to me