Hi! Yes, he and Iris do have sex, though not all the time and only when he wants to as he prefers other forms of showing his love. Seo-jun identifies as demisexual (an orientation under the asexual umbrella), a sexual orientation where he only experiences sexual attraction after forming a strong emotional bond with someone. This is hinted at when he mentions his ex, when he says that he didn't feel comfortable doing it with her so early on in their relationship.
Asexuality does not mean abstinence. Just because an asexual person doesn't experience sexual attraction to someone does not mean they won't engage in sex. It varies from person to person though. Hope that clears up the confusion!