the one who is smiling with Shiratori and then we can see his back with a bite in his nape
http://www.mangago.me/read-manga/kurui_naku_no_wa_boku_no_ban_beta/uu/aef4f166b58d3be52672b100fe5f26f5_chapter-1/pg-21/
http://www.mangago.me/read-manga/kurui_naku_no_wa_boku_no_ban_beta/uu/ibx_chapter-3.2/pg-25/
utou was always plan A is his head, but because of the misunderstandings he tho his feelings were one sided- makes sense- so he wanted to not burden him. the only reason he is still with the alpha is because of the way it was handled after sumi and Utou kissed. so it was thrown out there, lets stop this, and I hope for you to be happy with alpha. SO EVEN IF EVERYTHING WORKS, HE WAS ALWAYS PLAN A. and sumi isn't the type to go to Utou and ran back to him, the while issue was stop burdening, so he would think that he would be burdening him again
I hope I make sense
I know he loves Utou and he never loved Shiratori, they don't even sleep together and he is delighted that nothing happened between them. But he also decided that he wants to make things work between them and now he is crying. I really want to know what he is thinking on right now. In some way or another, all three of them are pitiful.
I understand your point. once I read a oneshot where I don't know if it was because of the lack of pages but it seemed like the girl had stayed with the protagonist because the love rival was bad. But I think that here Utou and Sumi weren't ready for a relationship, even without the bite mark, cause they weren't able to fight their fears and doubts, they would may have ended up breaking up really bad if they have became a couple at that time
I belive that that's what Sumi is referring to when he ask why they can't choose, not to the lack of options but rather they feeling unable to choose because their lack of confidence in taking that decision

Maybe the omega in Shiratori's memories dumped his ass after him not getting the job, or maybe he was forced by his family to bond with another alpha because they thought that Shiratori was not worthy of belonging to his family after having his job oportunity snatched by a beta