What a beautiful artwork. I love this duo! Last Order and Accelerator are so cute. I don't think of them as lovers at all, though, but more like familial. The author himself emphasizes as much in the afterword for v22: [link]
"From the bottom of his heart, what Accelerator wants is not a lover but a family. In spite of knowing nothing about what a family is, he's still been able to act as a parent for Last Order (though it’s unknown whether as a father or a mother), although there are some contradictions with how he acts as a child around Yomikawa and Yoshikawa (most prominently, the scene in Volume 15 where Yomikawa takes the gun out of his hands)."