Romance is taking a back seat.
I might try to weave potential romance options with semi-plot importance, but it depends on how well I handle the exposition of their plot relevance. If I fall flat on this, I’ll either just scrap the romance idea or acknowledge that the MC is in a relationship but won’t include any “ground breaking” scenes starring the MC and their romance interest.
Most importantly, I want to avoid making romance like something out of YA novel, or just avoid YA novel romance cliches because, let’s be honest, they’re not realistic in the slightest, they’re overused, abused, and sometimes silly…
If there is going to be a romance option, it’ll probably try to resemble Root/Shaw, a.k.a. Shoot, from Person of Interest. Here’s some off the cuff yet good tumblr posts that, I think, explain how Shoot is a quality example of a good relationship: here and here.
In this story, the MC and their romantic interest will be - or at least, I’ll try to write them this way - two complex, independent characters. I want to avoid defining the romantic interest as just a romantic interest with no plot relevance - it’s nice, sure, but it’s not good writing in a plot driven story. 
What I want to try to do, is create other characters - which the MC can enter in a friendship, romance, a sexual relationship, or something else altogether (I’m running out of adjectives here) - where they’re the hero of their own story and the relationship between them and the MC is just a portion of their life.
Going back to your question; if there is a romantic/sexual option who happens to be a demon, then they and the MC could have a physical relationship if the demon is Flesh Bound because they already have a physical body. If the MC seeks a relationship with a Spirit Bound demon, there’ll probably be a lack of a physical aspect to the relationship because they lack a body.
And no, I’m not going to curtail the narrative to where a Spirit Bound RO changes their class - becomes Flesh Bound - just for the MC, because that action has an unhealthy undertone, one that I don’t want to promote.