why adam wanted sex in first place??

so eve can keep ‘wanting’ him

Cuz a chapter ago or two. When the other Espers were hurt, Eve refused to let Adam guide them, saying they became useless, and if they're useless it's better they die. That's probably why Adam did that, cuz he didn't want to seem useless to Eve. And that's probably why Adam got hurt in the first place cuz he was frustrated with the was Eve thought, so he went too hard guiding all the other Espers alone, he couldn't guide Eve at the right time, n consumed all his powers to end up hurt like this. So in short, Adam didn't want to seem useless to Eve now.

so eventually if he didn't guide eve he will become useless?

Someone explained a different thing

Well, that's how Eve thinks about the other espers. And Adam is probably afraid Eve gonna think that about him too now.