Responses
I do agree with you that he made it clear from the beginning but to me in order to be a sex friend with someone it actually requires some type of friendship they weren't even that. He was just someone he had sex with. They didn't have anything outside of that. I don't think DG cared so much about the sex as he cared that Alex didn't give a damn about him. I think that was his problem. Like you don't have to love him but at least somewhat care which Alex does but DG doesn't know that and for the love of god neither does Alex. I just think they aren't sex friends because they aren't even friends.
I really do not like Alex as a character. However, since the very beginning, he told DG that he wants to have a mere sex friend, nothing more. Then when he started to think DG could have feelings for him, he reminded him that he only wants a sex friend and that he can't be more to him. From the very beginning, Alex was totally clear about this topic, he even asked DG again whether he is okay with it. Since DG agreed, I don't think he has the right to blame Alex that he doesn't treasure the time they spent together.
Ofc, Alex is manipulative, rough, selfish, egoistic. He can be blamed for his bad behavior towards DG but in particular this "breakup" case, I do not think he was at fault.