Again, friends with benefits does not equal a committed or exclusive relationship. He can date, check out, flirt, or sleep with whomever he wants and so can you. Sure it is a bit rude to be blatantly doing it in front of you but he is single and even people in committed relationships check out other people.
Please find a therapist to help you if you aren't already in therapy. And then join some sort of activity, hobby, or volunteer somewhere where you might meet someone interested in a relationship and not just hooking up. You don't seem to be cut out for hook up culture and that isn't a bad thing but it does mean you should look for something that isn't going to cause you so much distress with unequal expectations, that isn't fair to you or to your partners and you could be so much happier.