from direct MTL and recent chapter context, he doesn't love LOVE her. but he does become aware he has started developing a crush on her (caring/interfering too much about her safety, asking kael about her when the convo doesn't involve Hes, making excuses to meet up with them just to talk to Hes when things with Diana get worse), and then thankfully he realizes what he's doing and puts a stop to his feelings it on his own and sets boundaries.
It was more necessity. Than a personal choice, really. What ultimately ended the marriage was, Diana didn't want to or couldn't bring herself to do it. The most important duty of a crown princess, to produce an heir. Helios as a man would have waited and given her time, but as crown prince. He couldn't wait.

Can't wait for Crown Prince to come crawling to the party and get rejected by Hestia, I hope the manhwa draw out the scene nicely ლ(´ڡ`ლ)