I can only say the way I explain it to myself. Sebastian isn't able to consummate a relationship because of who and what he is. It's not about Hawke's decisions for once, it's not you, it's him. Apparently, while he once delighted in the pleasures of the flesh, that's not who he is anymore and whether Hawke likes it or not, he can't change. If a chaste relationship isn't Hawke's cup of tea, she should look elsewhere. Whether she chooses to encourage his acceptance of the life he's been given or suggests that he resist it, in no way will he be able to be physically intimate with Hawke or anyone else. If Hawke struck out and Seb slept with every tramp in Hightown, I'd be upset. The way it's presented isn't hard to head canon.