Nathan hadn't so much as considered having a family after his father died. He didn't put too much thought into it as a kid but sort of vaguely assumed he would because that was what people did.
After everything fell apart it was completely off the table. Especially post-Brianna when he knew he couldn't even be in a relationship with someone because of his power.
Even without his power…he was a mess. He doubted he would ever stop believing that his mother's last curse was true. The fact that Sierra knew his deepest shame made it worse. It was a real mystery how she decided she was still in love with him after hearing the awful truth.
Nathan deserved to be alone so how could he accept the love of someone so untainted? He couldn't do it. He just couldn't.